DivX Encoding and BITMAPINFOHEADER biSizeImage value

I try to find out the meaning of the biSizeImage value in a video encoded with DivX.

Identical recording settings produce videos with different biSizeImage values on the very same machine.

Here is what I’m doing:

I record and encode from a live stream (video camera, video only) to an avi file.

This are the encoder settings:

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-b1q 8.5 -r 640,480,6 -pre 2 -dir "C:\Program Files\DivX\DivX" -pq 7670 -b 3 -nf -deinterlace=1 -thread_delay=1

With NTSC:

Most video clips have an image size of 1036800 some are producing an image size of 691200

With PAL:
Most video clips have an image size of 829440 others are producing an image size of 1244160

This makes it impossible to append this videos without re-rendering.

I patched one video and changed the image size from 691200 to 1036800 and it had no impact on the playback of video. This made it possible to append the video.

So I’m really interested to find out what this image size stands for.

Does anyone know where to find more information?
Does anyone know why I have different image size although the encoder settings are the same?

Thank you for your answers.