Vitamin D is a fat-soluble vitamin. When fat individuals get vitamin D (whether from skin, food, or supplements) much of it gets squirreled away into fat stores rather than in circulation. As a result, fat people are far more likely to be vitamin D deficient than smaller folks.
This makes the paper Vitamin D Status and Cardiometabolic Risk Factors in the United States Adolescent Population (from September 2009's Pediatrics) particularly interesting. It concludes:
Low serum vitamin D in US adolescents is strongly associated with hypertension, hyperglycemia, and metabolic syndrome, independent of adiposity.
I wonder how much of the supposed metabolic risk of being fat is really a result of vitamin D deficiency? The evidence is just emerging, so nobody knows for sure yet. Still, fascinating stuff.