NEAT

Maybe it's a no-brainer why nervous, fidgety people tend to be thin... They move more and have tenser muscles (like unintentional isometrics) -- and maybe they're too antsy to spend much time eating. This article (from the American Journal of Physiology) indicates that a person can increase their energy expenditure 20-40% above normal by fidgeting. Now I have a good excuse for moving in my seat during lecture!
The article discusses various aspects of nonexercise activity thermogenesis (NEAT) and their collective role in weight control -- you might find it interesting.

I can just see it on a weight loss plan... "...30 minutes of aerobic exercise... 4 hours of foot tapping, 3 hours of leg bouncing, 1 hour of facial grimaces..."