It has several factors to me:
*'Style' of Routine- Since small is deep, it's not about necessarily the flashiest skills, it's about who hits them the best. Since large has such a strong depth of field, you need to hit hard in ALL areas, or so hard in one that it eclipses the others by miles, giving you more 'wiggle room' in points. Some teams have routines that might look better with less people, or better with more.
*Ratio of skills to size- Think of it as 'trimming the fat,' particularly in the jump from large to smaller divisions- a team who does fairly well in large, but doesn't win a lot might be better suited to medium, so that their ratio of skills competed to people on the floor is higher.
*Execution- Having to do with whether or not a team should be in a worlds-bound division or lower, depending on whether or not they really 'can' do the skills.
*'Style' of team- I rarely pay attention to some gyms coed teams, while others I hardly notice their all-girl. Others I watch equally. It's like that year that Cheetahs was a small coed- just never felt right. Same as people saying they miss CF in small senior, or when Panthers went from Large to small. They might still do well, but team dynamic/presence/style might make them seem 'better' in a different size or style (coed vs AG)