So, small breasts covered with scales is nudity...

But big breasts covered with a tiny cloth, is not? See here: 
http://www.renderosity.com/mod/gallery/index.php?image_id=1815516

And some more big breasts "painted" with a second skin, is not? See here:
http://www.renderosity.com/mod/gallery/index.php?image_id=1815467

Both these images show the "covered" breasts in the thumbnail and are not flagged nudity.

Where's the difference? In the color of the paint?

You have to admit, that's really funny... :-)