too tight?

so, my boyfriend has a much-bigger-than-average penis, and he says my vagina's really tight. Not painful, just tight, and uncomfortable if we go for too long (I feel the same about getting smashed for too long with his dick as big as it is). I know that vaginas are supposed to loosen up during sex, and I've heard that if it's tight, then "he's not doing it right" or whatever, but I can be fully aroused and ready for sex and even orgasm and I'm still really tight. Is this bad, or do I just have a small vagina? Lmao