I think all logical types are simply not as good at smiling on command as ethicals. But in general, EPs are much more demonstrative, and try to overcome this by being silly or over-expressive when the need for smiling arises. EJs, I think, tend to look down right uncomfortable about it.
Note that this is not always true for genuine natural smiles, like when someone tells a good joke or when you are with loved ones.
The two guys in this picture have Fi PoLR. Notice how they over-exaggerate Fe in a completely retarded manner, while using Ti to mock it at the same time.
Center-left is my SLE brother. Also note that he uses gross motor skills to exaggerate, whereas I (far right) use finer motor control (you really can't see that in this blurry pic) to imitate the behavior.
This pic contrasts XLE and LXE. We are actually both trying to look "natural" (lol) and smile normally. We both suck at it.
My LIE friend on the left has moved his face a whole lot less, and doesn't really raise his cheeks when he smiles. His gaze has a more "serious" look to it, and he seems unwilling to get into it. I, on the other hand, look pretty happy but also pretty goofy, with more of my face being involved in the smile than seems necessary.
This is just some examples of things I am personally beginning to notice in VI. I'm trying to record my observations more regularly so I can present them more clearly.