The reason DRKs feel more squishy is because they don't have a mechanic built into their "tank" stance like the others do, or don't have a shield.

For warrior, you get the 20% added heal bonus that's built into their tanking stance. If they didn't have that, they'd be just as hard to heal or even HARDER to heal than DRKs due to their overly large health pool. PLDs don't have a mechanic built into their tank stance, but they have the added armor and stats of a shield which neither of the two other classes have. DRK has none of these things, and it's what hurts them in regards to dungeons.