creating movies from still images

Hello,

I try to create (using mencoder) movies from a bunch of images,
but I found some anomaly I cannot explain.

I have two sets of 3000 images (jpeg). The two sets are exactly
the same except on was resized to quarter size (half each dimension.)
The fr is the original size, and the hr is the reduced size.

$ identify fr/00001.jpg
fr/00001.jpg JPEG 480x384 DirectClass 2kb 0.000u 0:01

$ identify hr/00001.jpg
hr/00001.jpg JPEG 240x192 DirectClass 1kb 0.000u 0:01

Here is the disk usage for the directories:

$ du fr hr
73380 fr
33548 hr

I use the following command to create the movies:

$ mencoder mf://fr/* -mf fps=25:type=jpg -ovc lavc -o fr.avi
$ mencoder mf://hr/* -mf fps=25:type=jpg -ovc lavc -o hr.avi

My problem is that the two resulting avi files are almost the same size:

$ ls -l ??.avi
-rw-r--r-- 1 vilmos vilmos 12099726 2006-03-24 15:22 fr.avi
-rw-r--r-- 1 vilmos vilmos 11711996 2006-03-24 15:23 hr.avi

If I dump the images to png from the avi files, then the full
sized is four times as big as the other, which is what I would expect.

BTW if I use different compression methods, then the filesizes
are far bigger (sometimes even 10x as big).

What's going on? How it is possible that the avi created from
the images which are quarter of the original ones is almost
as big as the big ones? Did I miss something?

Thanks, Vilmos