It's not hard to find the main file, since it has to be named index.html and nothing else. The issue is twofold: one, a safety limit that prevents someone from e.g. uploading a zip bomb and causing a DoS on the server side, and two, a common sense limit so players don't find their own browser and OS taken out by a game too big to load and play. It happens often enough to me as it is, with games way under the limit.