Well, I doubt either of us have statistics for what really drove its success (and if you do, I'll gladly cede the point), but anecdotally, I and a lot of my friends at the time switched to Gmail not because we needed more storage (I don't remember even thinking about that as a problem with Hotmail at all) but because the interface was way better. If anything, I think it had more to do with the "cool" factor of getting an invite and the cachet of Google at the time than with the storage. But I freely admit this just personal experience talking.