wait ... sorry, major noob question...
So i chose H.264 compression, then "best" quality... and what do you mean by 720p... that has nothing to do with he size does it?
If you don't have Quicktime pro it makes it much more difficult to get good quality with a small MB memory. Just torrent that shit if you don't feel like buying it.
Also Quicktime pro will help with exporting 10x more.