m0useanswered grow question 8 days ago once it switches to flower the plant can double in size, if its not that big to begin with then it won't get crazy big.
Autos a a crap shoot, sometimes they work good, other times they don't. not the same as photos and better off just growing photos and controlling the flip to flower. Can also grow photos from 12/12 and they act like an auto but gives you the flexibility to veg em out if you need to.
Some plants don't develop good roots structure and stay squat. and autos roots are sensitive. if they hit dry soil they think its the edge. if it to wet they rot, root stress can cause early flowering and stunted growth.
Next time try photos