The BG maps themselves were absolutely huge.
A good example is the full Saradush map I pieced together some years ago for the hell of it (I was going to do every map, but it was just way too much work)
http://img.photobuck...es/bgmaptry.jpg
This is drastically jpeg compressed, so I got it down to a little under 1 MB for a 3395x2882 map.
The full PNG file however, is 16.2 MB.