Ray is right. No data structure, especially flat ones (i.e. disk-based), are without some amount of overhead. In Maemo Mapper v2.0's case, the overhead is in storing the "keys" that are used to find and access a given image in what is otherwise a sparsely populated matrix. This overhead, however, is significantly less than that of the internal fragmentation of typically formatted memory cards.

The images will be stored in the database as they are retrieved (from Google Maps, Virtual Earth, etc.). This is usually in either PNG or JPG format, neither of which reacts at all to zip compression, because they themselves are already heavily compressed.