The more you can understand about the Admissions process, institutional goals and other political realities, the better you can deal with the difficulties attached to the end result, over-crowding. Ask questions of your supervisor if that information is not forthcoming. Make sure your supervisor understands the specific issues with which you are dealing. Be clear in your communications, but do not become a complainer. While people who make decisions need to know the realities as you know them, you are expected to handle your responsibilities and you will be judged by how well you handle the challenges.
Enrollment management is an institutional issue and decisions are made on that level.
In addition, it can reflect poorly on your department and on your supervisor. Everything is a challenge and attitude is everything. You'll be surprised what you can learn about yourself and the issues at hand when you attack overcrowding from a positive perspective. Yours can be a particularly complicated position when over-crowding occurs in the residence halls. On the one hand, your primary responsibility is to see that room assignments occur in a fair and timely manner.