I don't know what is causing your file to be so big. I tried saving a tune like this, and it was 12kb. Saved it as a JPEG, it's 47 kb, but it will probably look a lot better.
I think Sue probably gave you good advice. Changing the size of the GIF or the compression would help. Also, cropping it to just the tune and getting rid of empty space and borders will help. I haven't found an easy way to do this and agree with Joe - it would be nice if the program would automatically convert the tunes.