Sometimes my boyfriend will say that my vagina is looser or bigger when we haven't had sex for a couple of days. He thinks I'm having sex with another person whose penis is bigger but he is the only one that I'm having sex with. What could it be?

His imagination is bigger than his penis apparently.

In my sex life if my wife is more aroused her vigina is more gorged with blood, the same way my penis is therefore making her vigina seem tighter.