redbaron, i was just about to post something similar.
the only problem that I have with the body type things is that it's not really real-life applicable IME. I'm 5'10", weigh around 125 lbs, and have like TEXTBOOK ectomorph body type. I'm also incredibly loud and rambunctious - you might want to say
- for an INFJ. Two INFJs I know are average weight and rounded. They're very very quiet and very
.
If I were to make a hypothesis, I would say that it's not related to body "type"- ectomorph, endomorph, mesomorph- though I think there might be a correlation there for various reasons.
In general, I think that anyone with a low body fat percentage will seem naturally more introverted because they take up less physical space and have, arguably, less energy to exert. Very thin people are also "frail" in a way that looks introverted because their body doesn't have enough fat to function properly- i.e. the person is frequently cold, tired, pale.
The exception I would make to this "thinner = more introverted" rule is when the person associates thinness with a good personal image. The increased self confidence will make him/her appear considerably more extroverted, not introverted. So, for example, the times when I've gained weight I've become really shut off and reclusive.