Compare the 128x128 and 48x48 icons, and then compare the 48x48 and 16x16. As you go from 128 to 48 much of the detail in the fox's fur goes away, and as you go from 48 to 16 it becomes a simple gradient and the detail on the planet disappears as well.
If they simple resized a vector image, the 16px would be a muddy mess.
(There are probably much better examples somewhere, but this is just the one I've used in the past- apparently, for the last 10+ years...)
Near the end of this post:
http://www.hicksdesign.co.uk/journal/branding-firefox
Compare the 128x128 and 48x48 icons, and then compare the 48x48 and 16x16. As you go from 128 to 48 much of the detail in the fox's fur goes away, and as you go from 48 to 16 it becomes a simple gradient and the detail on the planet disappears as well.
If they simple resized a vector image, the 16px would be a muddy mess.
(There are probably much better examples somewhere, but this is just the one I've used in the past- apparently, for the last 10+ years...)