Re: SIZE and MASS, works in progress
Revised Density Increase: +3 MASS and +2 STR per 3 Character Points.
Under these mechanics, the extra MASS itself is what effectively provides the PD, ED, and KB Resistance on the Power, based on the Relative SIZE and MASS Scales of the attack and the character.
Without it, you would just gain the STR benefit, which is more expensive on its own that a straight STR Characteristic boost (1.5 Character Points for each STR with Density Increase) so an Advantage to remove the gain in MASS would be pointless, or a +1/2 Limitation instead (making it equal to STR.)
"No Additional MASS except for Damage Scaling and Knockback" maybe as an Advantage of +1.
Based on the current Density Increase, the cost should be equal to the +STR, +PD, +ED, and +KB, after an Advantage removes the penalties for the weight.
If every step of 5 Character Points Density Increase gives +5 STR, +1 PD, +1 ED, and -1 KB (9 Character Points, if each paid for separately), then that means the doubling of weight each step is worth a -4 Character Point return, so a +1 Advantage for "No Additional MASS" to Density Increase is about the same - infact, a little worse - than if you bought those stats individually.