Did you just revert to your old .gmk file, when you realized GM Studio wasn't worth it? Or did you port it to GMStudio, and then port the GM Studio project back to Game Maker 8.1?
In the case of the latter, Game Maker Studio just generally uses more space for resources, such as images or sound, since it has to be able to go cross-platform. By taking a Game Maker Studio file, and reverting it to Game Maker 8.1, you inadvertently port the larger files, resulting in a larger file size. If this is the case, the only ways you can get it back to it's original state, is if you have a backup (.gb1 / .gmk), or if you re-import all your resources from their original sources.