Thanks so much for a great suggestion. This is great feedback. I do want to point out however that when we do have larger listing photos available, we do display them. It's not as simple as a mouse-over but almost. If you take a look at the listing below as an example, you'll see a link under the main photo that you can click to 'View larger photos'. This link will bring you to a new page where you can see larger versions of all the photos that are available.
Keep your eye out for this link when you search. It's not on every detail page, but we're reaching out to our listing sources for more and larger photos so you should start seeing it more often.
I hope this is helpful. Please don't hesitate to let us know if you have any more feedback.