When uploading an image via the media uploader, WP automatically makes at least 3 additional sizes, that comes to 4 images including the original one. Most of the time, the images will be used at most 2 ( 1 thumbnail for the archive page and 1 for within the post itself. )
Before long, the site will be bloated with unused images, and that's ok until the time to backup site.
Suppose a new WP site is up and there will be 1 post per day with 1 image per post, for a year, we will have 365 times 4 image files sitting in the upload folder ( 1460 files).
Within 3 years, we will have 4,380 image files all together !!!
So how do you handle this ?