Not a good description :) but this is what I get:
1. I use printDefaultSizedImage to display the image
2. I upload only images that are 600px longest side
3. I set upscaling to none
4. I set max image size longest side also to 600px for galley and albums
5. I set image quality to 100 for the large/middle size (although I don't have/want a middle size) and the thumbs to quality 70.
Now I can see that although the images are 600px and 50kb in size when uploaded, in the cache folder there appears a cached image of 600px, 140kb in size!
Three years ago I had this problem. Have I forgotten something?