WordPress has a function that allows a theme to set how many sizes and how many thumbnails a image should have, other that the thumb set in Settings » Media. And, upon upload, thumbs are created.
I understand why the theme needs to make some thumbnails, for better design. I do not understand, however, why this need to be an absolute pig with the thumbs. I took over a WordPress blog of a buddy, and the theme created 5 thumbs for every image. FIVE! FUCKING FIVE!
And the developer was being an absolute pig: 50×50, 150×150, 300×300, 600×400. Fucking hell, man! If I upload an average of 100 images a month on my blog, the space will be over fast and stupid.