But you have opened up a new can of worms - two files with same "pixel resolution" can have dramatically different file sizes? Is that because different (lossless) compression algorithms are used by different programs?
The "same pixel size" does not necessarily mean lossless compression. Only if you are using a lossless format like TIFF or RAW, but a JPEG can have any file size at a given pixel size, depending on the "quality" value.
A JPEG is representing the image file by a mathematical approximation, and the quality slider is specifying how accurate the approximation will be. A lower quality will need less coefficients for the approximation and result in a smaller file size, but will still have the same pixel size.
I notice too, for example, when i exported a 7.9MB iphoto jpeg with max full quality, it showed up in finder with a 15.7mb file size.
That can happen, if you edit a JPEG, that originally had a medium quality setting, and then export the edited JPEG at maximum quality. The file size can increase, when the JPEG will be rendered at the highest quality and all JPEG artifacts in the original image will be perfectly approximated. With JPEGS it really helps to know the original quality values, because it would be futile to export a JPEG of medium quality with the highest quality settings. That cannot bring the original quality back.