I think it just all depends on the school size. At my school, with around 20,000 undergrads, a sorority size of around 25 wouldn't survive. There's just too much that sororities have to participate in, too much work to keep a chapter alive at a school like this. You need a proper support size, which average at around 90 does. But down the road, at Newberry College, their average is around 25 (I think) and their sororities do fine. I think that greek systems at different schools, for the most part, know what's best for their school as far as chapter size.