Comment 10 for bug 497728

commit :
http://git.gnome.org/browse/nautilus/commit/?id=b0d808de8c8d1be6b8a4403fc587cc032612027a

This was changed after being discussed during the Nautilus UI meetings. Anything smaller makes it really tough to actually see the content of the thumbnail itself.
However, there is also a proposal to increase the size of the icons too.