The bore area of a 12g is 75% larger than that of a 28ga.
Put an ounce of shot in both & the column will be 75% longer.
To move that "Mass" with the force applied to only 57% of the area requires either a higher pressure per Sq In, longer burn time or in most cases both. Yes per square inch the longer column give more inertial resistance to moving. That doesn't contradict any of the "Laws" you have cited. I may well be a "Dumb Country Boy" & not as highly educated as you are but I am Not totally Ignorant.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra