I don’t understand what exactly influenced such a small size. Tell me what can be improved

Stupid autoflowers.

As mentioned, that lamp is problem nr.1. i think that low humidity in combination with high temperature is too not good fkr that little plants. And sharing container...i never seen it before. But who knows. Someone has to try it.

Not enough light or not enough water, and that constantly. She was starving so much, that she got used to it.

2 plants sharing a pot doesn't help Yellowing from bottom up = nitrogen deficience, but that minor issue would not cause catastrophe. Are these autos? grow photos. can mitigate mistakes with more vege time.

Could be any thing from to compact medium to how it dries out.. temp slightly high humidity is to low.. If you need advice on your next grow message me il be happy to help.

The cheaper 600w HID lamps work great for under 0.

Start making diaries. Hard to provide much accurate help without the full picture of what you're doing.

As mentioned, a quailty full spectrum LED light would be the first change I would do. Would love to see a diary so we knew more about your grows such as medium, nutrients, etc Some strains stay small, genetics plays a big role. Training is a must! Topping, LST, SCROG, etc will all increase yields. I would stick to Photoperiod types for now, you control when they flower so gives extra time for growth, training, etc. Autoflowers will flower without changing daylight hours so they give much less time for maximizing your space. Look at diaries of other members and see what others are doing, particularly the strains you plan on growing.

Now nothing. In next grow - light would be first thing to change. Mars hydro, vipra or spiderfarmer has cheap lights, they work 10000 times better than those bluepurples. Plant looks a bit overfed and in lockout or food provoded in wrong range of ph, she cannibalize herself , most probably because cant uptake from pot.