Solved Why Massive Difference ...

[MourningStar]

... in size of these two image files?!!??!!!!

They are almost identical in pixel (2850x2850) & dpi(600) & bit depth (24). I frequently encounter this phenomenon and I finally want to know how to make the larger file to be similar in size to the smaller one.
(could it possibly the 'information' in the image itself?)

Note : On my system CD.jpg is 4.25 MB and CD_Back.jpg is 762 KB.

 

[Alejandro85]

Simply because the two images are massively different (in terms of content) and you're using a compressed image format (jpg).

Jpg, as well as any other compressed formats, store images not pixel-by-pixel, but by compressing it so that they occupy as little as possible, but compression algorithms have better results when the source has a lot of repetitions and is as uniform as possible. The top image has lots of small details, texts, prints and small variations all over the place, while the bottom one is pretty simplistic in that aspect, hence the file size difference.

On the other hand, if they were saved as .bmp (an uncompressed file format), they would have exactly the same file size, down to the byte (provided they're the same resolution and bit-depth).

This is exactly the same reason why standard compression programs (likie WinZip, WinRar and the like) obtain far better compression ratios on text files rather than on binary files. Text files use few characters, with certain repeating patterns, while binary files use the full byte range with some unpredictable contents, resulting in higher final file sizes.

 

[MourningStar]

thank you Alejandro85 for confirming my suspicion.

 

[DavisHill]

Alejandro85 explained it. We usually open and save as .jpg any photos to reduce the bytes capacity.