Hi Shivz,
I don't really know how the A2 samples, especially for VGA or movie modes. Without reverse engineering the firmware it's only a guess - but with the burst speed that is necessary for 30 frame per second VGA capture, it's unlikely that the movie modes use the same process as still captures. Whether or not the 7 frame per second captures in the VGA mode are analogus to movie mode is another issue. Remember, all cameras capture a RAW image then convert it into one of several possible forms in either compressed or non compressed modes. Most RAW algorithms being used today are a compressed tiff file which accounts for the differentiation in file size between the RAW and uncompressed tiff modes.
It's entirely possible that the A2 may simply ignore all but the center 640x480 pixel matrix for the VGA mode, but this would seem to be counter intuitive since that would effectively constitute a "crop" and change the field of view accordingly. Since what you see in the viewfinder which is electronic TTL appears to have the identical field of view as when shooting in higher resolution modes, it would appear that the camera is still capturing with full resolution then downsampling.
Still captures on the F828 are made with full resoluton and down sampled before writing to media. This same process is used in many digicams and in most dSLR's which have multi-resolution capture modes. As for the VGA movies I haven't a clue. It would seem to be counter productive to do it this way since most are progressive scan.
Best regards,
Lin