I'm making a custom folder icon similar to the ones in the Home folder (Pictures, Documents, etc).
To do so, I extracted the images from the .icns of the blank folder icon and photoshopped my own graphic onto them. I followed Apple's guidelines for icon sizing. I then packaged all the images into an .icns using Terminal.
Now here's the problem: When I put my icon next to one of the home folder icons (with icon size set at 64 x 64 in Finder), it doesn't match. For my icon, OS X is displaying the 32@2x icon. But for its own icons, OS X is displaying a scaled down 128px icon. If I re-export my custom icns without the 32@2x size (so OS X is forced to use the 128 instead), it looks fine.
Here is a screenshot. The difference between 32@2x and 128 scaled down is subtle but definitely there.