Oh dear, now I feel stupid. It was indeed the background that was bloating the file size (I had a static color in dapper and have gfx in edgy).
** Changed in: gnome-utils (Ubuntu) Status: Needs Info => Rejected -- gnome-screenshot increased file size in edgy https://launchpad.net/bugs/70173 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs