If you’re hitting her cervix during sex, that can be painful. I can attest to that. My current partner is well above-average I’d say, and sometimes it hits just a little too hard. If I were you, I’d suggest different positions that might make her more comfortable with vaginal sex. You can Google them. If penetration hurts even without you hitting her cervix, I would think a trip to her OBGYN would be in order, just to be safe.

Also, I will also say that while I don’t prefer anal over vaginal sex, I do enjoy it significantly more with my current partner even though he’s much larger than anyone I’ve ever been with.

You should talk to her about this more in-length to get a better idea of what’s going on.