Drawing Size?

My picture for Fruits Basket is saved as a JPEG image file. Its size is 2,548 KB.

My "Supergirls" is saved as a GIMP image and its size is 3,587 KB.

My "Food" is also saved as a GIMP image and its size is 1,311 KB.

My other file, "CastleandButterflies" is saved as a GIMP file and its size is 2,148 KB.

My last file, "Cow", is saved as a GIMP file and its size is 197 KB.

The pictures that were saved as GIMP, I originally created them at school on Adobe Photoshop CS2. When I burned them on a CD use the CDR-W, and opened them up at home, they were saved as GIMP files, because my home computer doesn't have Adobe Photoshop CS2. (It has Adobe Photoshop 7.0. )