Please explain why does penis size vary in different environments?

Penis size variation: The size of your erect penis is dependent on several factors. The most important is your degree of sexual arousal. The penis swells more the closer you get to orgasm. If you measure your penis when you first become erect, it will probably be smaller than at a moment near ejaculation. Viagra (sildenafil) increases size by increased blood flow. A flaccid penis shrinks in cold to maintain body temperature.