The other day I realized I had accidentally set thumbnail generation in WordPress to be exactly 150×150 even when that wasn’t a proportional resizing of the larger graphic. Ugh. I reset the pref to the proportional setting, but what to do about the existing thumbnails?
Well, this is WordPress so you have to use a plugin for everything, and it turned out there’s a nice Regenerate Thumbnails plugin that will let you resize all thumbnails after changing the preferences. Worked like a charm for me.