Read here - <http://tangentsoft.net/video/mpeg/edit.html>

The above link explains some of the format of an mpeg file,
particulary how you get a movie into a smaller format

When you go from AVI to MPEG you lose information, and going
from MPEG back to AVI does not replace that information

If you think you will EVER need to edit your original again,
you need to store your original AVI files

Some ways that have been suggested here are...

1 - export finished AVI to a new (or 2-3) DV tape
2 - write finished AVI to as many DVD's as needed
(this of course means in 4 Gig segments)
3 - export finished AVI to an external hard drive