Nero Vision Express 2 (why does it expand MPeg and Video Clip files?)

[Segageo64]

Why does Nero Vision Express 2 expand a video clip or MPeg file thats only 437,517 kb to over 2.5 Gb in size. I've used Ulead Movie Factory 2 and that program doesn't have to expand the file to make it playable on a DVD-R so why does Nero Vision Express 2 do it?

If I use a file thats 750,000kb or so and over Nero Vision Express 2 has to reduce quality just to get it to fit on a 4.7 GB blank DVD-R disk.