What filetype are you expoting the video as? If it's an avi then that's why the video's too big; go back into your video editing application and export it as an mpeg, or a mov if you can

If you're already exporting it as a compressed file format, like mpeg, then your codec settings are probably biased towards image quality rather than filesize. If you reduce the image quality then the size of the video file will go down, but compress it too much and it'll look like arse