Lindsay,
I am not experienced with cauliflower, in fact never had much luck with it around here. With that being said, I do remember reading that you are supposed to tie the leaves of cauliflower up around the developing heads to keep them white. Something about the sun discoloring the heads. That would explain the leaves being so big.
Not sure if the same holds true in a green house, or not. I would give them a bit more time to see it the heads increase in size. If after a week or two, and they still haven't grown any more, I would probably put something else in there. Good luck.
Blessings,
Winona