How well a image data compresses depends on image content. Image with High detail do not compress well image will little detail will compress well and the file size will be small. Compare a image of a blank white wall to a wall with a black and white checker board wall paper. One is all white it white no other detail to detail the other has squares that vary in size because of perspective angle and distance a lot of detail must be recorded.
I do not know why they can not open some of your images. File size should not be an issue. All your image decoded are the same size Width number of Pixels Height numbers of pixels background layer only for these are jpeg files.
All that is true, but the OP had (in my understanding) compressed by JPEG six times the same
source image (about 15MB) with the same settings, but he got nevertheless two sets of results:
6.2MB and 3.2MB.
Therefore one should think about the parameter settings. Were they really equal? As far as I
can see, there is is one degree of freedom left: embedding or not embedding the CMYK profile.
A 3MB (large) CMYK profile might be possible, but standard offset profiles are up to only 2MB.
Best regards --Gernot Hoffmann