[HoneyBadger]

I think if you have a 5-6 length weapon, it should be effective against calvalry, regardless of how big you happen to be. If it's 4 or less, then yes it can be size-determinate, but I don't see any reason why a huge heavy guy armed specifically with an anti-calvalry weapon should be penalized against an enemy he should-by all natural rights-have an advantage against.

Yes, I realize that it can be taken advantage of, but why not mod lances in the process so that they hit larger creatures more easily? And give war-horses the ability to independently trample smaller units at the same time their riders are attacking, while still retaining the ability to kick. That would even things out and make better sense.