Featured images - why do they come up hoge on mypproduct, page or post

here's my question today - I'm not saving them up, honestly!

Not sure this is just about marketpress, but it's where I'm encountering the problem. If I add a product featured image that is of any reasonable size, it comes up huge on my product page. The only way I can see to display it as a thumbnail is to make it very small in the first place, thus negating the lightbox feature because there is nothing bigger to see. How do I solve this?