I have a similar question. I have been capturing AV through a USB port using "AV to USB2.0", "Microphone (4- AV to USB2.0)", and "720 x 480 [3:2], 30 fps, YUY2" .
The last 2 videos captured were both 2hrs, 3 minutes... but the file sizes differ significantly. File 1: 10,361,534 KB; File 2: 5,887,905 KB.
I am at a loss to explain this behavior.