I think Google Maps feeds you bitmap-like images, while something with fully-loaded maps is storing vector data. A complete street map made of bitmap images (of any compression type, eg .jpg or .png) is going to be a lot larger than a map made of vectors.

Even in vector form, a US map is several hundred megabytes. I don't know how big a full download of Google Maps would be, but it would probably be many gigabytes even just for a regional map.