Well the original question was why are they small.
They could be small because of genetics. Some strains grow short regardless of how hard you try to make them tall.
Beyond genetics, it is on the grower to maximize growth. Looking at the only pic in post #1, you have no obvious issues with nutrients, watering or lighting. It is growing fine imo.
The issue then is how/why they grew like this vs bigger.
It is not the amount of nutrients. They are well fed. Your water input is/was also fine looking at your picture.
If this were a photo plant, it would grow until you flip it...meaning you would be able to veg it longer until it is the size you want, then flip. That alone is one of the main reasons many of us avoid autos. Genetic quality tends to be the other (the auto genetics tend to dilute the more potent photo genetics, but more and less depending upon the particular cultivar and source).
So what could you do in the limited time to improve growth? I just finished some autos myself this past Summer, that's why I took the time to comment again.
The time is limited, you have 2 to 4 weeks to veg the plant and then it stretches and starts throwing pistils and stops stretching and goes full flower.
In soil, you just have to get the wet dry cycle and environment optimized - ideal temps, rh, airflow, light and then water it properly and only as much as needed...dry out fully, repeat.
Another way to have bigger plants is to grow autos hydro, such as in coco. They grow faster this way, and so with limited time is the preferred choice.
Lastly, you can give them 24 hours of light through veg to max out the growth, and switch to say 20 4 or 18 6 only in flower. More light time = more growth. It is limited when we grow photos because of their genetic requirement for 11+ hours of darkness. Autos allow for more lighting time.
You did fine. You can do better next time. Your attitude about it imo is great. Hang in there and let them finish in their own time.