Guys know they’re not big. They just wish they could deliver that fantasy. Partly for her, but also for themselves. And yeah, getting compared to another person is not ideal if you’re insecure about that.
This is why I brainstorm silly sex magic. The sentiment is valid, it’s absolutely THE top shelf compliment for a penis. Perfect size and perfect use? There’s nothing better than that. But it’s fun to imagine protection magic that lets you play with crazy sizes and go nuts with what insane orgasmic impact a greatcock has on someone.
Anyway, this is easily handled with a little therapy to work out body issues and internalized harmful attitudes about size fetishes. But that’s the boring advice that really works and absolutely should be followed. There’s also the FUN way to handle this: He will forget everything you said if you demand you observe his skills in a threesome with women.