Here's an example mesh where the UV shells have a tendency to pack tightly together. Cheers! https://dl.dropboxusercontent.com/u/104089847/BevelExample.obj
I think the problem is that you try to unwrap a very high res mesh with a lot of really tight edges. You should relax those edges or unwrap it differently, for example you can cut it in parts and unwrap as strings rather than circles. Unwrap itself works as it should, it's your mesh is at fault here.