Pot size determines growth?

[NorCal Gal]

IMG_2604.jpgI'm trying some dense planting this year and have 52 varieties of tomatoes growing (along with peppers). I started all of these on Feb 7 and this is what they look like today. (Tomatoes on the left and right, with peppers in the middle.) Each cup/pot holds a different variety and I used what I had on hand, but for some reason, the tomatoes are way bigger in the green pots vs. the Dixie cups. They get the same water, light, and care (all started in ProMix). I sowed 3-4 seeds in each Dixie and 8-12 in the green pots and plan on potting up very soon. I will probably be ditching the Dixies in the future with these results, but I'm curious if anyone knows why the vast difference in growth?IMG_2604.jpg