The size looks pretty good for solo cups. I would pop a plant out and look at the roots. They might just be ready for a new home. Also, im not sure, but are you using sunlight from the window. If you are than they look just as good or better than most of my houseplants. You know after looking at the pics, i just think they need to be transplanted deep into a larger container and given plenty of light. They look like they had to stretch a little as seedlings. The old fan trick strengthens those stems too. Id check out those roots. They will be easy check because you were smart and used a solo.