Well there are many things that contribute to the filesize of a map. Mostly imported resources. Any unused resources (icons, models,
music, random useless crap that you imported by accident but didn't realize it
, etc.) that you have imported into your map should be deleted.
Oh, and music is a big one. If you have any music imported into your map that you don't need, you should delete it. Try not to use extremely large sound files, as they usually take up a lot of space.
Map preview images usually don't take up that much space, but I guess if it's big enough it could take up a little bit of space. That probably isn't a problem here, since you had that all along.
If you imported A LOT of useless crap, that could also do it. But really, importing models and icons to use for your map isn't bad enough for the map to gain approximately 10 MB over night.
I don't really know how the filesize jumped up that high, but if I had to place a bet on what I think happened, I'd put my money on imported music files.
Oh and before I forget, try using some
BB Codes to spice up your description when you get around to improving it.