AutoGK - XviD to XviD to reduce file size

[marioval]

I have a 400MB XviD file that I was going to burn to a CD with some other files. All the files will not fit so I wanted to just reduce the size of this video just a little bit. I fed it into AutoGK and set the quality to 90% and left XviD as the codec choice and everything else was set to auto. I was thinking I would end up with a file slightly smaller than my original since I would be reducing it by 10%. I ended up with a 600MB file so obviously I did something wrong or I just don't understand how AutoGK works. What exactly does that 90% mean? I ended up using AviRecomp and just choosing 350MB as the output size and that worked great but it took to long and I didn't need 2 pass encoding. For future reference can someone explain what might have gone wrong with the AutoGK conversion? Thanks

[manono]

...but it took to long and I didn't need 2 pass encoding.

Yes you do need 2-pass encoding. Whether with AviRecomp or with AutoGK or with any other encoding method, the only way to ensure the size you want is by using 2-pass encoding.

[marioval]

I'm not debating that. I'm just trying to understand the logic. I started with a file, I told AutoGK to give me 90% of that file, I ended up with a larger file? It just doesn't make sense to me..... Also when I Convert DVD's to XviD using DVDFab, I choose "DVD To Mobile", I select 1 pass custom size and choose my size and the resulting file is pretty much always exactly what I chose, give or take 50 or 60MB. I don't bring that up as if to say "i'm right and your wrong", just adding as much info as I have to get to bottom of my question.

[nic2k4]

It's pretty simple to understand, when you set a quality value in AutoGK, you're telling it to encode at a fixed bitrate. Doesn't matter what the input is, the output will remain at that bitrate. If your input file had a lower bitrate and you encode at a higher bitrate you will end up with a larger file (not necessarily better looking). If you had a specific filesize in mind, you should've set AutoGK to give you that file size. It would have encoded in 2 pass, but the second pass doesn't take that long. One caveat, you get what you get when you re-encode like that.