My (very briefly) Gargantuan Wild Shaped Dire Wolf salivates at this thought.

Math time!!!!

Lets say we have a Dragon born with Volume=v (v=xyz), Mass=m. It's Density would be d=m/v. We also know that it kick ass with a Force=F, where F=m*a, being a the acceleration of it's arms while punching.

If our DB double it's size in every dimension, it means that it's Volume is: V'=2x2y2z > V'=8(xyz) > V'=8V.

If we assume it's density keeps constant, then it means it gains some mass.

D=m/v, but now D=m'/v' so

m/v=m'/v' , mv=m/(8v) , m8v/v=m' , m'=8m

If we assume that it will punch with the same acceleration as always, F=m*a

F'=m'a, F'=8ma F'=8(m*a) F'=8F

So, changing from medium to large would imply changing 8 times the strength. If this keeps up for every step, then we are talking about one extra change to get to huge, and then one to get to gargantuan . So 8^3 times the force. And if we assume a linear relation between force and damage, 8^3 times the damage...

Yeah... A few extra d4s seems.... Meh