Any Way To Really Determine Final Size After Encoding?

I have an older system - a P3-733 w/320MB memory and a Seagate Barracuda
7200 RPM HD and it seems a tad slow, but works for my needs at the moment
as I am just learning multimedia things. I can capture using MPEG2
through my TV-Card, but VOBS still seem a bit of a mystery through the
DVD Player, which I just picked up.

After several tries I find that ripping the DVD using DVD2One works best
at maximum compression, and then rip the Streams using TMPEGenc and
convert the AC3 to MP3 using BeSweet and remultiplex. This results in a
two hour movie fitting on two CD-R's nicley as I have a CD-RW but not a
DVD-RW.

Main question though is that after I bring it all down, I find that if I
use Xmpeg 4.5 to convert to AVI and use MPEG-4v3 as the codec, it will
generally bring the file down so it can fit on a single CD-R, with
respectable looks, using 768 as the BitRate, stripping the MP3 to Wave
and then using VirtualDub to encode the Wav into a MP3 with perfect sync
and Direct Stream for the video.

Still it has me baffled as I did Silence of the Lambs for example which
is about 2 hours and one minute and BatMan forever which is about 2 hours
and five minutes, exactly the same and BatMan came out at about 620megs,
and Silence of the Lambs came out at about 480megs - both being 320x240
and using Media Player Classic to play back at full screen setting its
function to use 640x480 as resolution for full screen, and both are darn
good in terms of a very crushed movie.

Cannot find a calculator for MPEG-4v3 that is accurate at all. Is there
anyway to actually know why the files are so drastically different in
final size - or is this just something Xmpeg does itself?

Thanks.