There is a multitude of factors that can affect penis size. Genetics plays a large role. Cisgender men have one X chromosome and one Y chromosome - it's the Y chromosome that determines the development of the penis and testicles. However, the Y chromosome might not always carry the gene for penis length and girth - after all, the X chromosome contains far more genes. There are a few other factors as well. It's possible for hormone imbalances during pregnancy to affect a fetus' genitals, as well as imbalances in the fetus. Epigenetics may also play a role - this is when a person's environment affects the expression of their genes, which can have effects such as influencing hormones. Nutrition is another factor - malnutrition at any point can affect hormones and development and can even delay puberty, which can contribute to a smaller penis and testicles. Certain factors can obscure the penis and make it appear to be smaller. Both body fat and pubic hair can make a penis seem to be smaller than it truly is. To add to this, cold temperature exposure can cause the penis' blood vessels to constrict, shrinking the penis temporarily. You may have also heard myths about other factors affecting penile length and girth, such as too much masturbation, as well as height, and the size of you feet. These are all urban legends that aren't actually true, so you can rest assured none of them will contribute to the size of your penis!