Contrary to popular belief, a large root mass does NOT necessarily equate to more nute uptake = better bud mass. Low Pressure Aero, DWC, and NFT roots are often reservoir clogging huge, but they lack efficiency. Look at those root pics you will notice that they look like a mass of swollen spaghetti noodles. Sure they grow nice plants, but so much more is being harvested from smaller root structures, where the environment is favorable to the root developing lots of side branches (as seen in the AP video), which CAN then lead to root hairs.
Root Hairs are very tiny and look like fuzzy cotton candy. The amount of surface area is exponentially larger than LP/NFT/DWC roots.
The most efficient root structure is that which develops in a High Pressure Aeroponic environment. I have been using HPA for 5 grows and when all the parameters are dialed in (easier said than done), HPA out performs any other method. I have a journal that started out as all HPA, but due to some unexpected timer issues was going south fast, so I quickly put together a F&D rig, then came across this thread so I bought some APs. My journal is now comparing HPA v Flood & Drain v Air Pot. I moved one near death plant to a 4" AP and after a week of recovery and explosive growth I transferred it to a 1.5G AP. The roots were very similar to HPA roots. AP is a whole different ball game for soil growers. I pulled that plant as it was a male, but was very impressed with how thick the main stalk and primary leaves were. It's actually outside as I plan to cross it with my F&D female. She is 4ft tall and about 3 feet wide, looks like a Christmas tree. Her leaves look very Indica, but she is growing like a Sat. The male looks pure Indica, so the cross should make for shorter plants that are smaller versions of the mother.
Most soil growers use pots that are too small for the size plant the amount of bud they will harvest. Until AP, the rule of thumb was 3 ft plant needs 5G pot. Based on what I saw with my male plant, my gut tells me you can easily grow a 3ft plant in a 1.5-2G AP, That is a huge advantage for many stealth growers, plus the root structure is superior to anything short of HPA.
I have a bunch of clones rooting right now, so my next test will be all the same clones in the 3 different mediums. All will be under the same light as I am using now: a Quantum Bad Boy 8 bulb HO T 5 mixing Quantum Bulbs with coral bulbs. hth