One thing you may want to check is if both the orginal images have EXIF information. One senario is that Picture 1 does have it and Picture 2 doesn't, possibly stripped by the photo editor. If it is present it can actually take up more bytes than the image on smaler images.
See: http://suffandnonsense.blogspot.com/2006/04/reduce-thumbnail-size-in-ez-publish-by.html for more info.
