This is spot on IMO.
I think too that if the roots once hit the plastic they get a stiffle in growth because from now on, always one side of the roots faces plastic which is inert... so the root cannot utilize its full potential.
I consider running a nearly completely closed container for the next organic grow. No drain needed but I'm gonna install just a few glaspipes in order to see when the water reaches the bottom. The proper amount is easily to calculate. I can't even remember having problings with overwatering at all. If you grow large plants they can deal with water more eassily, and under strong light the evaporation from the leaves increases. I'm using quarzsand and expanded clayballs on top to prevent the topsoil getting dry, keeping its temperatur more homogenous between days'n'nights, and more humid. I did consider to close it completely with but then I would need an automatic watering system.
It's nice to see how roots grow when they have a high air humitidy aroudn them - I've seen them even grow healthy exposed to light. Thank you for the info about the correlation of fanleavesize and potsize. In this case it's the more the better. You can use the opposite (terribly small cups) for a cut you just want to save the genetics/small slow motherplant. Combined with week light.
edit:
Cannabis also develops a different root when grown in a larger, expecially deeper container. Outdoorplants develop a thick main root. This does not happen in small indoor cups. Once the downspiraling roots hits the bottom it'll initiate a second root and grow more bushy generally. Outdoor it's more straight with less sidebranches and more like being described by science text/wikipedia.
Hi
@Kassiopeija !
Thank you for your comment, this is one if my favorite topics of growing in general, and I feel like most people agree that a large container is especially important for anyone who grows in soil or organic based soil blends, if not hopefully this thread will help more people start to realize the benefits of going larger if you're able.
One of the major benefits of going with a large container that I've found, is that it allows the grower the ability to train the roots to search for the water down deep into the container, if the container is wide enough it can be watered indirectly every single time it gets water, and what I like to do is switch sides or water diagonal corners of the container at a time, and switch between sides when I water, this way the plant gets some cycling and not just the same water from above each time, this helps train the roots and the bottom root growth will be reflected back on the top portions of the plant in leaf growth and branching, perfect for healthy clones and typically you'll have a larger plant that doesn't mind supercropping and training as much as a smaller plant would in a small or medium sized container
(Note: I realize some plants don't need as large of a container, any strain that doesn't get real tall or wide can probably get by fine in a smaller 7-10gal container, this post excludes those less space consuming strains, helps to have a larger growing variety to begin with if going this route, if they are smaller varieties, 2 plants can grow in one large container just fine, you'd still see similar growth to growing just one large, me personally I grow basil on either side of my single plants in the 27gal, large containers are good for companion planting also (just get a trouble free herb or plant that deters pests like basil), if anyone's seeking options).
I hear about the topic of air pruning the roots often and I do see the benefits of it, however I do not feel air pruning matters (as much) in a larger container, air pruning is important if the space is limited because has less space to extract it's nutrients from the soil within, if there is less space, you'd want the plant realizing the dimensions of the containers and creating fine hair like roots that spread as fast as possible, this is why air pruning holes in the airpots/fabric pots and similar designs work so well for air pruning, and why it's useful.
But for a larger container the plant is more at its own leisure to grow out where it wants to, it's not as much of a race against time for the plant to find the nutrients that it needs, in a larger container more of a constant flow of nutrients can be uptaken by the roots because within a larger network of roots in the soil, there is less trouble seeking out various nutrients & more natural control over deficiency, just seems that the roots have an easier time bargaining with the soil when in a larger container, in simple terms....
I liked what you said about covering the containers, I feel like the reason it helps is because it creates a high humidity level above the soil and below and the roots seek out that moisture even if it leads them straight up out of the container, lol, and the new root growth is typically mirrored in new growth above the stalk, in the leaves. Sealing that moisture in some allows the plant to fully utilise the full expenditure of the container also, if only the bottom 70% of the soil retains most of its moisture then the plant can't utilize the rest of the drier 30% towards the top of the container as freely as it could, having the need to do so..... my plants don't seem to think the container is too large, and they seem to enjoy a partially sealed top layer ( I do too, I don't have to water as often and when it does start to dry out its a slower occurring process, less plant stress if lower moisture levels are reached).
In effect covering the container to hold in moisture causes some air pruning right out of the top layer, just by keeping the roots happy and moist, just be sure some air exchange is still allowed, roots need air almost (slight exaggeration) as much as the leaves do.
But yes, you are right, it does seem much more difficult to over water a larger 20+ gal container, I'm sure It can be done, but me personally I like to have that assurance just in case I do over water then I will get some run off, so I do kind of like having one drain added to mine, one option might be to add the drain higher up so that more water resides in the bottom, I put mine about 2 inches above the bottom and in 27gal of dirt that's still a lot of residual water that is left to reside in the bottom without runoff.
But yes i don't think there's any reason to doubt what the reason is between fan blade size and container size, it's rather simple? if a plant is allowed to grow a large foundation of roots below the soil, it will reflect in the size of the rest of the plant overall, and It does appear to be seen by the size that the fan leaves reach and the overall amount of them, makes for a faster growing plant from what I can tell
.
I'm sure you already know this, but one thing anyone growing in a larger container should be mindful of is soil aeration when combining a blend of soil that is to be added, organic matter, perlite, volcanic rock, sand etc, what ever you choose just be sure that the soil with have some lightness to it, it should have a lot of density to it that will hold moisture, but not allow air to penetrate through it, if air has a hard time penetrating the soil substrate, then so will the roots.
One final thing I'd like to mention that has made a large difference for me in my grows so far, is adding crushed lump charcoal to the soil, it seems to be what was missing from my soil all along, the plants I've added maybe 8% crushed charcoal to the mixture have shown vigorous growth and seem to not mind the charcoal being there at all, seems what people say about charcoal being a good host environment for microbes to live from within is true, seems to help provide benefit to their survival, which then adds in the survival and thriving ability of other organisms, such as
plants living in the soil nearby.
I had decent luck crushing the charcoal with just a simple 3-4" screw drive type clamp, as long as the charcoal isn't super hard wood like mesquite to begun with, just a hammer, a clamp, and a mid sized pipe wrench pliers were all I used to crush 40+ lbs if it into powder so far, and yes I got blisters from the work and sore hands, but it was worth it in the end, my plants are growing bountifully so far. Just be sure and wear a good dust mask, maybe some eye protection also, do it where you don't mind dust settling nearby if you do it indoors, even with the added effort, I would do it 10x times over if I knew I'd see this large of an improvement beforehand.
So thank you for reading this (I'd be glad to explain further if any Q's arise), I hope this helps someone, and that more growers Will be able to benefit from the words left here today for them to follow and build upon, I hope it helps people grow lbs for themselves, I really do... take care my friends.
IF you want your plant becoming big and full in its growth, more = better. Rootboundess is a stifle. 
