I'm a skinny one as well (5'11", about 9st (60kg-ish?))... however, i've known people with suspected Aspie traits who were fat, so i don't think it's a 100% correlation... however, i *do* think that there are probably physical traits which are genetically linked to AS (but that not *all* people with AS may also have) - for instance, hypermobile or otherwise unusual joints, weakly expressed secondary sexual characteristics (relatively little facial hair, or small breasts, etc), thick hair/lack of male pattern baldness, and a particular, sort of long/angular facial shape...
I think fast metabolism could possibly be one of those features, or it could also be fidgeting/stimming burning up calories (IIRC, studies have shown that fat people are more likely to be able to actually stay still, whereas thin people tend to move about, even very small movements such as shifting position while sitting at a desk or watching TV, all the time... I certainly find it almost impossible to ever sit or stand absolutely still)... also, i read somewhere recently that the brain is one of the biggest users of energy, and so being stressed can burn off calories just by the brain being "overactive", so typically high levels of stress in AS may play a role as well...
I think there's also a possibility that being such a picky eater (and consequently eating such an unbalanced diet and relatively little food overall) as a child may have affected my skeletal and muscular development (much as Katharine Hepburn was so skinny all her life because of the malnutrition she suffered as a child in wartime Holland, IIRC)...
On the other hand i think i definitely have metabolism issues which exacerbate my AS sensory issues and vice versa, particularly when it comes to hunger, tiredness and cold...