Converted HD MP4 to SD AVI. Filesize increased!

[avihalp]

Input: 1280x720 MP4 (80MB). Output: 720x404 AVI (200MB)!

How can this be? I know that MP4 has better compression, but the downscaling from HD to SD should make the sizes about even.  Why did it increase so dramatically?

[Jan Gruuthuse]

it all depends on the used video codec to re-encode to avi. Some codecs have all frames present and less compression/frame.
Example video encode in RGB: 1 sec is 178 Mib

[avihalp]

I do not know what this RGB is suppose to do. But how do I reduce the size of the converted file? Because 200MB for a 3 minute video is ridiculously large.  Especially when it is standard definition.

I used the Xvid4 codec for the conversion. And never has the sizes been this disproportional.

[Jan Gruuthuse]

Constant Rate Factor?
2.5.6 = 26
2.6.0 = 20