Help desperately needed, recompressing Quicktime movies

[RentaLL_MAY]

Hello everybody,
this is my "First Contact" with this wonderful forum.
Hope i can get some help with my "quicktime-questions".
I already tried to find help with the search function but had no luck. Most of the Quicktime related question are about conversion to or from quicktime.

Actually i need help with recompressing Quicktime movies.
First question would be if there is some tool similar to Virtualbub where i could set only the video track to be recompressed but keep the audio track untouched. Just like the "direct stream" option in Virtualdub.
Reason is that i need to make some quicktime movies smaller but have to keep my hands away from the audio track.

Second question would be about recompressing quicktime movies.
It seeems that either i am to dumb to get it done or quicktime is not very good in that field.

My problem is that i can't set "Quicktime Pro" to recompress my files at a given data rate.
Here is an example:
Let's say i got a movie that is 1 GB big which i need to make 20 % smaller.
Let's presume that the movie is compressed originally at 4000 kBits /s. So to make it 20% smaller i enter 3200 kBit/s (which would be 80% of the original kbit/s) into the "limit data rate" window hoping this will reduce my file to 80%.
But this does not happen.
After exporting the fiel is still apprx. 1 GB huge. So i go lower with the data rate but filesize still remains almost as huge as before.
Then i reduce data rat even lower, maybe at 50%, hoping that this might reduce the filesize at least close to the aimed size.
But now the file becomes very much smaller than needed.

This is driving me nuts.
Setting a certain filesize and data rate seems to be a total matter of guess and try.
What am i doing wrong?

doing the same task in DIVX is so much easier. Here you set your desired data rate and all works fine.
Why is it so terribly unpredictable in Quicktime?

[Baldrick]

What video codec are you using? Cause quicktime mov is a container. Or maybe it's not possible to select codec with quicktime pro, was a long long long long long time ago I used it.