I have heard of fraternities imposing membership caps on a chapter basis. Or I think sometimes it is from the national (don't know why, but it sticks in my head that Delta Phi does this). The reason the guys usually do this is because in their belief, the smaller numbers make for a better and tighter brotherhood.
I don't think a national sorority would ever dictate a member cap as a NATIONAL policy. You've probably heard people on here talk about "total" or "ceiling" which would be different than a cap. If a sorority is at ceiling of 40, they can only take quota. If a sorority has a CAP of 40 and they have 35 going into rush, they can only take 5 pledges, no matter what quota is. I hope this makes sense!