Hmm, this one has me scratching my head.
When you use categories.php to upload a product image, the image file size is increased dramatically. Some examples from my store would include a jpg of 39k jumping to 134k. A jpg of 31k jumping to 117k.
My originals are larger than my largest image setting, so I assume their physical px size isn't being increased. As best as I can tell, they are displaying in the pixel size I created.
Other resized images in the products and thumbs directories are inordinately large as well though obviously still smaller than the images_big files.
I'm surprised as well to see that it isn't reported anywhere else. I have an unmodified 2.5.3 installation on my development server and this behavior appears there though less dramatically. The 38 K jpg mentioned above increased in size to 96 K on my dev server. My dev server is windows, so basically, I'm going Windows to Windows rather than Windows to Linux as when uploading to my live store.
Create a product, choose an image with the image tool in categories.php (I'm using the browse feature), then save.