The vagina itself is a muscular tube of about 8 cm that, when adequately stimulated, expands to fit any size penis with ease. When your partner initially penetrates you, the muscles of the vagina contract and grip the penis As you continue intercourse and become more turned-on, the vagina expands even further – sometimes so much so, you can’t feel his penis inside you no matter how large it is. This explains why for both partners the initial few thrusts are sometimes the most pleasurable because the vagina feels tighter. (Tracey Cox)