My bet would be that many users are saving automated runs with tons of screenshots movies etc, does it only seem to be one project? I would do some investigating to see what users are saving and what is contained in their runs. The best way to cleanup space is by deleting run results, you can purge old run results and then the next time that the garbage collection runs on the project they will be removed. Another thing that takes up a TON of space are libraries and baselines. The first step to understanding how to keep this project from growing is going to figure out what the users are doing to create this much data, additionally, the only way to safely delete stuff is through the UI or by connecting to alm through the FTP as described below:

http://eyeontesting.com/questions/4548/where-are-the-attachments-stored-in-almqc-11.html

Using the FTP lets you actually see what is inside the repository instead of just seeing the numbers with no logical extensions, you have to be really careful when deleting via the FTP there are folders for tests, attachments, etc, I would recommend deleting from the UI so you don't accidentally get rid of something important.

My bet would be that someone is creating libraries and baselines of automated tests in mass, this would make entire copies of folders of entities on the backend for that specific baseline which is a ''snapshot in time of certain entities'' other than that the only thing I know of that would make a project grow at this rate would be running thousands of automated tests a day and saving screenshots and movies to the repository while doing it.