Yeah, you can grow some pretty big plants in small pots if you feed them right. Just look at a hydro setup, look at the root mass, and imagine it all packed in a small pot. It takes alot of roots in a small space to become rootbound. On my first grow I used 2 gallon pots and ended up with 4 foot monsters, after vegging them for like 6 weeks, and topping them a few times. When I pulled the roots out of the pot, I was surprised that it wasn't super-rootbound. The roots filled the whole pot, but there was plenty of room for more growth. I probably could've grown the same sized plants in a one gallon pot, maybe smaller.