From any damaged frames of high caliber tubing that I've seen, the damage isn't from rider weight, but it's always been some sort of crinkle/crumple/dent damage. Thinner walled tubing is easier to damage in a front-end impact with crinkle damage to the downtube, or dent damage from a hard laydown if the tube impacts something (I've seen a 953 frame dented from tipping over into a picnic bench when the wind caught it just wrong from where it was leaned. Flip-side; I've dropped my '88 Trek 400 on a parking block and barely done anything beyond scratching the paint. Could be coincedental or bad luck/good luck, though.)