I think Traysea made a good point........expand the main level if you can. However, I have limited room for a cage (2x5) and I find my 2x2 loft to be invaluable. My two don't run around the cage very much so no matter how large the main level might be, I don't think they would utilize the space. They get f-a-r more exercise with running up and down the ramp than they ever do with the occasional zoomie.
The ramp takes up little space and is positioned where the piggies can run on both sides of it. Some people constantly mention how difficult it is to clean a cage with a loft. I just don't get it! I have utilized an offset/overhanging loft and find it very easy to clean under it........despite the fact that my cage is wedged into a corner between the wall and my desk.
Having a 2x2 loft creates the equivalent of a penthouse apartment for the cage. The loft has a large litter box with hay rack suspended over it, water bottle and pellet dish........and a cardboard house/tunnel. My kids seem to enjoy having a space where they can get away from each other for privacy. And, they take turns sleeping either in the loft or downstairs.
Suggestions from everyone are helpful, but you ultimately have to experiment and decide what is right for your situation.