(I'm just talking out of my ass but) I'd imagine the screenshots can be compressed pretty efficiently, as the content of any photographic detail like images/picture don't need to be preserved with outstanding quality..
Yeah I bet there's a lot of stuff you can do to greatly reduce space usage without even nerfing resolution or losing color. For example if you are always using the same desktop background you could save that once and then just store a reference to it for all subsequent screenshots where it also appears. Also, window chrome and things like webpages and IDE windows will compress well, since there are so full of long runs of the same background color
