DOWNLOAD FILE SIZE

I'm kinda at a loss here...
Trying to upload a demo for a game of mine, but for some reason, I can't include ALL of the files necessary to run the game. Each time I try, I get hit with a "File Size Too Large" error message.
Is there some sort of way to reduce the file size, or submit the download file I wish to put forth? Or do I just have to give up and scrap the whole idea??
This is honestly VERY annoying...