Penis swelling

I'm in a long distance relationship and when my boyfriends at home he has a big penis but when he's with me his penis swells and gets thicker and longer, he notices it too. I find it really strange as when I am his house we can have sex without it hurting too much (I'm quite tight so sex always hurts to some degree) but when we are at my house sex is so painful as he has "grown" does anyone know what can help it or why it happens?