as for equating mass to being more resilient to knockdown--it's a creative suggestion, but it doesn't really reflect reality, even on an intuitive level. being larger/taller is a disadvantage in many ways in regards to being knocked down; it's actually kind of a foundational principle of hand-to-hand combat. of course, this does only apply to bipeds--it's hard to argue the logic of something like a four-legged wemic being harder to knock down.
While this almost sounds logical, except on the intuitive level, we are talking large mass (over 500 pounds) size difference. Use a whip to trip up a halfling or human maybe, but a giant that weighs 5 times what you do and is built to leverage that mass well? We aren't just talking tall human proportioned creatures here. Even in human terms think of merican "pro" wrestling, how hard is it to dropkick someone like andre the giant or push him down if you dont have the mass to counter his mass? A marilith tailwhipping a 300 pound human (ripped stats plus armor) is still a big difference than trying to knockdown a 12 foot tall 1200 pound half giant. High mass tall creatures have the weight advantage, and are built to leverage it unlike tall humans, and if you can't leverage your weight against that yer judo is less effective than it would be on a human. A fat weak person is harder to move than a strong but more fit proportional one. We aren't just talking human proportions here. Relating Kd resistance to size isn't just thinking in terms of human proportioned heights, sorry if there was confusion in the correlation.
Ya just tossing out an idea, thankful for all the work yall already put into the mod. And with all the kd checks being out there, combined with how str oriented big races are, figured an anti kd bonus was the most flavorful and intuitive counter to large races getting penalties and being stat for stat point worse than medium let alone small races.