Small roots = small buds. Try using a bigger container to grow in. Autos do great in 3 gallon pots. Also consider your 600w LED is probably not 600 true watts.
She's almost ready to harvest from the looks of it. I'm guessing her growth is pretty much done. You may be surprised on how dense it comes out though.
If you want bigger, better buds then evaluate your grow method and techniques.