Which camera are you using, S3IS? That sounds like the CHDK functions that get all the dimensions aren't fully implement. This is common, because there's a lot of variation between cams. If you confirm which camera and firmware version and are willing to test, I can probably fix it.
That is correct, the Canon PowerShot S3IS. It is my prized possession.
My current CHDK Build is CHDK 1.4.1. Revision 5167. FW Vers: 100a. Compiler: GCC 4.9.3.
My CHDK PTP is chdkptp-r964-win-x86_64
This is most likely due to the incomplete implementation. If this is one of the cameras that does 720x540 live view in video, you should be able to get that over PTP at least. Though we don't know it's actually captured at that resolution, it's conceivable Canon could be capturing at 640 and upscaling to 720 for the display.
I don't know... When I did the rmem dump thing, some of those buffer images seem to have a width of "1848pixels" and a height of, around the same. It seems to look like it has a great resolution, although I might be doing something wrong.
Then again, I'm not exactly a genius like most guys here. I'm mostly specialized in dumping and reverse engineering 1980s arcade cabinet ROM data, and extracting image sprites. This is the same process that I am doing on the RAW data I rmem'ed from the camera.