These new dragons seem to have thresholds where their firebreath is stronger. I had one that was 30% dead... it breathed fire on me and I shrugged off the damage. When the dragon regenerated up to around 50% it breathed on me and I got hit HARD.
I'm not sure that other firebreathing creatures work like this or if there are just HP thresholds in the firebreath equation. I'm also not sure if the base damage on the creature impacts the fire breath or what. All I know is the breath damage increases dramatically at certain points of health.