Changing KFI does/doesn't effect file size

[otwist26]

Can anyone explain this to me? Without knowing in more detail how the divx codec works, I'm unable to figure it out.

I'm encoding 2 different video clips from HuffyUV to DivX 4.12.
One Simpsons episode and one King of the Hill episode.
Both are exactly the same length (21 min.)

Here are the resulting file sizes for a few different encodings:
2 pass encoding with VirtualDub
Audio is MP3 192kb/s
Video is 6000kb/s
All other variables stay the same for both clips except for key frame intervals.

KotH
KFI of 5 -> ~700MB
KFI of 4 -> ~730MB
KFI of 3 -> ~765MB

Simpsons
KFI of 5 -> exactly 780,404 bytes
KFI of 10 -> exactly 780,404 bytes
KFI of 30 -> exactly 780,404 bytes
KFI of 50 -> exactly 780,404 bytes
KFI of 300 -> exactly 775,360 bytes

Here's my confusion... Why does changing the KFI have such an effect on one video clip, but has none (almost none) on the other?


thanks!