I also can't comment on DI, but at DIII the only problem I have with guards is the ridiculousness that a 5 foot something PG with no blocking or rebounding ability is supposed hold his own against a legit scoring SF just because he has a good DEF rating. I agree there should be some height or strength penalty, not a rating, in terms of guarding bigger men. For example, if X amount of inches smaller or X amount of strength (this number would have to be added alongside height and weight and would be generally basically in line with a player's height and weight) weaker, then the penalty kicks in.
But to the OPs point for DI, I believe the Bearcats won a couple titles back well before I was born with 6-5 Oscar Robertson playing PF and C. Just what came to mind... different time...