dave2008
Legend
I'm actually his DM, and my concern wasn't with the overall damage output so much as with the fact the claws and bite base damage seem to stop increasing at Young Adult. So even though they go up a size category each time to Huge and Gargantuan, Claws do a base 2D6 and Bite is 2D10 still, which seems like they should go up an additional D6 and D10 respectively for each size category. Is that a typo or something? Just about everything else seems to scale with size increases.
I have a similiar concern, it just doesn't feel right. I am not really interested in a dragon that does more damage through more attacks, but that hits harder with each attack instead. That being said, I also find damage to be low in general for epic level threats (20+). I would bump up the adult and ancient as dictated by the weapon size guidelines. Thus, I think a simple fix would as follows:
Young Black
Bite: 15 2d10+4; 4 (1d8 acid)
Claw: 11 (2d6+4)
BW: 49 (11d8) acid
Adult
Bite: 22 3d10+6; 4 (1d8 acid)
Claw: 16 (3d6+6)
BW: 63 (14d8) acid
Tail Attack: cost 2 actions (to bring average damage in line)
Ancient
Bite: 30 4d10+8; 9 (2d8 acid)
Claw: 22 (4d6+8)
BW: 85 (19d8) acid
Tail Attack: cost 2 actions (to bring average damage in line)
It still feels low to me, but I think that keeps it close to the DMG guidelines ( I use my own)