I'm at a loss as that's what most of my friends with larger partners have done.
One thing you can do is breath out as they penetrate, that tends to cause you to relax. And use lots of quality lube. Other than that, just remind them that you giving your body to them is a gift that should be cherished and not abused. Sex that hurts in a nonconsenual way isn't fun for either partner, or at least shouldn't be. If they're getting any pleasure from intentionally hurting you then that's an entirely different topic that moves from physical to psychological.