The image dimensions are directly translating into file size. The game only uses Targa images with the very inefficient "runtime length" encoding. Using big dimensions on map images instead of increasing the zoom factor of the maps (it's not like higher resolution would make the image look any better) will result in quite large image sizes.
Yeah I guess so. I made the map a few months ago when I playing was around and figuring out I didn't have what it takes to spend the time editing maps.