Div X

[edding]

When I try to make a Backup of my DVD,I wonder about something,
when I use FlaskMpeg / VirtualDub:

From 2-3 Movies with the same Viewing format,sound Bitrate,length,etc. I get different results:

Using Div X Low Motion 1000 Kbps,1 Keyfram/sec,Sound 44,1 khz 128 Kbps (compressed later to DivX 64kbps)
One Movie has ca.700MB,one 900MB & one 400MB
Why can this happen ???