GIF images use a compression scheme known as run-length encoding. This scheme works extremely well at compressing large horizontal areas of continuous color. You can see how the file size of these sample images increases the further they stray from GIFs desired format.
956 bytes
1054 bytes
1050 bytes
1158 bytes
1588 bytes
1780 bytes
1029 bytes
1444 bytes
4563 bytes
10977 bytes