pixels, DPI and file size question

JPEG is a lossy compression format, meaning that the saved file, when opened, will never match the original pixel for pixel.  JPEG also supports different quality settings: the higher the quality, the closer it will match the original image (less compression artifacts), but the larger the file size.  Also, JPEG file sizes can vary significantly depending upon the image content.  The "busier" the image, the larger the file will be.

I suspect that the larger file, even though it has smaller dimensions, was a busier scene (more fine detail), thus resulting in less compression.