Canon S100 - JPEG file size too small?

This is probably an elementary question for most DPR readers, but I'm a little surprised at the small file size of the JPEGs I'm creating on my Canon S100.

The aspect ratio is set to 4:3, the image size is set to Large, and the Compression Ratio is set to Fine.

But when I shoot the same scene on both my G7 and the S100, the resulting JPEG file sizes are 3.9MB and 2.1MB respectively!

Seems strange that will all settings equal, the 10MP cam would have a smaller resulting JPEG than the 12MP cam. I assume there's something basic I'm missing here. Any tips?