I can export a file from PSD (photoshop) to PNG (portable network graphics) format.
With an image sized 1024x768 I usually get a PNG file that's approximately 120KB. However, if I open that exact same image with Apple's preview, and save it with no modifications, that file goes all the way down to near 40KB (one third of the original file).
Why is that? Is Preview losing some vital information? Is Photoshop adding extra information for internal use and Preview gets rid of it?