Your question says you're game is 15mb and you need 300mb... I think you mean the other way around?
Assuming a game is too big and you want it smaller, the options are varied.
1) If you are using huge images, and making them small, save them smaller to begin with.
2) Drop/Redesign if you have one or two massive files. Sometimes there can be a work around.
3) If you have large sound files, you may be able to convert them to smaller files. Granted you might lose some quality, but will it be noticeable?
4) Since you're in the C3 forum, when you export it, I'd also select the options prompted to repack etc.
Hope that helps