It is most likely because you have changed your "Website Layout" within the "Website" page of your settings. Changing a website layout is pretty much like redesigning your entire website, which means different image sizes. This also means that the thumbnails have to be of a different size as well. For instance the layout called "Slider Metro" creates the largest possible thumbnails, because it uses these thumbnails for slideshow on the homepage, while template "One Page Top Bar" does not need such slideshow, therefore we don't need the thumbs to be this big. Thumbing images is very important for your website quality and you only have to do it once per the template of your choice.
Weeee, you found me!
I'm your buddy Bottie, I was hiding behind the scenes, but now that you've found me I'd be happy to tell you what I'm doing.
I just wrote a few fun facts about IADB
Would you like to take a look?
Click here to check them out. I hope it will cause involuntary audible response.