by Ray Le Couteur.
I'm having the same issue with one of my courses
It's not the size of the file (I have restored a backup of over 400mb successfully in 2.5), but the database calls.
A relatively small backup with lots of data in 'feedback' times out. So it could be a database related setting.
But my most likely route (without tearing hair out) is to delete some old data and try backup/restore again.