Withouth looking at SS sizes... it depends on what the designer is using it for. Often they specialize in something and do the same type of work over and over - and know exactly what they need.
If buyers are downloading 300 photos per month - that is a lot of space.
I doubt many bloggers buy photos at SS, but the IS - XS is a perfect blog size - and the blogger doesn't have to open a separate photo editor and resize the photos before placing them in their blog. If they buy all their photos in the same size from the same agency the blog will have a consistency.