18MB file to 100MB??? Need help properly encoding this handycam vid pls

Well as the subject says, I have a handycam vid that becomes nearly 5x its original size when i try and enhance it.

The vid that I took was from 4th of july and it was just too dark. I adjusted the brightness and contrast using VirtualDub and it turned huge on me. I've dabbled with VirtalDub and Tempenc and I keep getting issues like this. I tried messing around with the different codecs that I have, but quality goes way down and filesize is still very high.

Usually I'll let some all-in-one software do all the work for me and i get reasonable results, but in this case, I cannot because of the framerate that the handycam vid comes in. Unmodified, the video comes at 30fps and for some reason my other programs won't accept it.

If anyone can tell me how to adjust the visual settings on my mpeg and keep the file size down, I would be very greatful.