I'd think the major factor, armor/toughness wise, would relate to size too.
Make a full set of armor out of Whale hide. . . .it's hide armor. . .
Hit you with a bat, it's just as good as cow hide armor. . .
But if you hit a whale in the side with a bat, it barely notices. . . .it's not the armor, its the 2' thick layer of blubber under the skin that acts like super-thick padding. . .
You'd need 2' of cotton padding under your hide armor to match the "Armor" effect of the hide when it's on the whale, since I'd assume you're not wearing the 2' thick layer of blubber also.
There's also the fact, that on the animal, the hide is a seamless, complete layer, not sewn together sections.
So I don't think there's much worry of thinking you could just rip +150 DB dragon hide off a dragon and wear it as +150 DB armor. . . .on the dragon it's integrated with the fat and muscle layers perfectly, melds to the body surface perfectly, each scale is perfectly anchored and integrated with the other scales, yet the hide has a degree of play over the body interior allowed by the subdermal connection layer that no armor underpadding can match.