In System Preferences > General, Lion has an option to change the size of the sidebar’s icons.
If you switch that between small and large, you’ll see quite a difference in how your Finder windows look.
Medium’s an option, too, but that’s just so…middle of the road. Pick a lane, darn it.
Ahem. In any case, this seems like what you’d expect. Sidebar icon size refers to the Finder, doesn’t it? Yes, but not just the Finder. Oddly, this preference also changes the size of the sidebar icons in Mail, so you can use this to tweak your message-viewing experience to your heart’s content.
Well, as long as different icon sizes are the extent of what your heart is looking for.