I don't know of anything.
People in the Middle Ages and Renaissance seem to have believed precisely the opposite:
SCENE II. A public place.Yond Cassius has a lean and hungry look;
He thinks too much: such men are dangerous.
Notice also the Reeve in The Canterbury Tales, who is "a sclendre colerik man", is ill-tempered and vengeful. He makes sure to pay the Miller out for his story.
The best of the four "complexions" (Sanguine, Choleric, Melancholic, Phlegmatic) in this view was the Sanguine Complexion, yet sanguine men and women are plump -- although perhaps the phlegmatics are the fatties.
The medievals seem to have seen thin people as likely full of fretting, nervous energy. I guess what one makes of such pop-psychology is another matter. There may be something in a picture of the soul that lasted for so long even if the medieval conceptions are quite different from ours and don't "map onto" any current conceptions.
I'm inclined to say, putting an Ancestral Health Movement hat on, that I think that someone who is properly nourished -- getting the right nutrients in the right amounts, not undergoing wide blood-sugar fluctuations, and so on -- is more likely to be calm. I guess such a person would be more likely to be at the weight that's physiologically normal for them. There must be so much else that plays into the picture, though. For example, it's been said that people whose jaw grows fully, so that their teeth fit properly -- rare in our society, owing to poor nutrition -- tend to be more relaxed. It could well matter at some level simply having plenty of room for your tongue -- not to mention that misaligned bones in the face can put pressure on nerves.
Then, of course, one's life experience is going to affect one's emotional tone. And there's the pre-natal environment ...
Human beings are so complex, I think it would be a difficult to sort out what does or doesn't have an effect on their degree of calmness.