Your metabolic rate must be high but then you might be young and "geared" that way through your genetics "roadmap".<br />
Why worry about it if you are healthy? You best advice for any health matter must come from your physician/nutritionalist.
I wish i new how to gain weight to im 6ft 5 18O i look like a damn skelton lol
Talk to a doctor about that. Some girls really can't gain weight (Iucky b****es, just kidding) so ask a doctor for recommendations. Eating a lot of crap won't help anything, but eating good things probably will.