It's a crazy little theory I have regarding types and their corresponding smiles. It comes from Pod'lair, but I think it holds some validity regardless. Basically, the more F you are, the bigger your smile will be. IxTPs tend to have a very contained, self-aware (Fe) smile, because they are Ti-doms and Fe is not immediate to them. Whereas IxTJs have a more open, unaware (Fi) smile, that's not as big as an F-type's (ENFJs have the biggest smile, for example), but bigger than an INTP's.
Crazy, yes.