Hi 2purrcats - I've noticed the same thing. For example, the editor displays both a 1024x768 image and a 800x600 image the same size. If I edit a 400x300 image however it displays the width about one inch smaller.
I'm sure larger images must be displayed at an optimal size to fit into the editing window. For example a 1024x768 could fill the entire screen or more (depending on ones monitor) if not downsized to a workable size.
400x300 must display smaller, otherwise it would be enlarged beyond 100% of its native size, with resulting badly degraded quality because of the "extra" pixels needed to make it display bigger.