why do sororities have limits on how many girls?

The campus ceiling (maximum number of members a sorority chapter can have) is determined by averaging the number of members in each house after Formal Fall Recruitment is completed. This is an NPC rule, and is recalculated every year. It keeps the more popular houses a fair size, while allowing houses under ceiling to COB until they hit ceiling.

On a strong campus, almost no house should have to COB the rest of the year after formal recruitment. Last spring, most of the sororities participated in COB, and some took sizable classes. If no one was needing to COB and the classes were getting bigger, that would indicate a need for expansion.

There isn't a limit to how many girls a house can take in a year, so long as they take quota (plus additions where applicable) in Formal recruitment, then bid up to ceiling the rest of the year. If quota puts a house over ceiling, they are done recruiting until they drop below ceiling, or until next Formal Recruitment.

Another thing that affects campus ceiling at URI is housing. Most of the sorority houses at URI sleep 40-60 people, so the membership needs to be enough to pay the bills. It wasn't that long ago that houses were having trouble filling some of their rooms because of lower membership, upperclassmen wanting to live down the line, and so on. So the classes being the size they are now are probably perfect for a university this size, with the housing currently available.