Append video file size too big ?

[tfodor]

Hello,
I try to append 2 MJPEG AVI video (with PCM audio) with direct copy, without any processing.
The resulting file size is much bigger than expected.
File A: 2 minutes, 240 MB (fps=29,97)
File B: 2 minutes, 225 MB (fps=29,97)
Appended file: 4 minutes, 697 MB almost 1,5x bigger than A+B.

Does somebody know why this happens, and how can I append more efficiently?
A+B = 465MB??
I have similar behavior with VirtualDub.

Thanks in advance for your help!
Regards,
Tibor

[mean]

Maybe overhead due to remuxing correctly
If that's the case, it would happen also if you just copy one single file

[tfodor]

Thanks for your hint, I made a test just saving one file with direct stream copy:
File A: 240MB -> 358MB
File B: 225MB -> 339MB
Adds up to 697MB exactly.
Well it is still a pain, what "correct" and "incorrent" muxing could be...
Any hint on this? The original video was created by a Car Cam DVR.

[mean]

They probably do one single chunk with all audio in it => overhead = 12 bytes
"Correct" muxing => overhead  = 12 bytes per ~ 40 ms

[tfodor]

Thanks for the hint, I made a try saving the AVI without audio this time to avoid overhead, but the file size still increased by 1.3.
I made a little analysis with VirtualDub -> Hex Editor -> RIFF chunk tree, and found that the resulting file has probably (based on their size) duplicate chunks.
Do you have any idea why, and how to overcome this problem?

Orig file: Start.AVI 4MB 60 frames, video 45 chunks, audio 24 chunks

VD save as direct stream copy, no audio:
Result file: Start_noaudio.avi 5.4 MB 60 frames, video 60 chunks (17 duplicates).

[mean]

Maybe frame rate is altered
That would explain the duplicates