Although there are some similarities, there isn’t a direct comparison in measuring vagina size and penis size. For starters, the vagina is part of the interior female genitalia, a canal for (but not limited to) childbirth, a penis, or other sexual activity. The opening to the vagina is part of the vulva, the external parts including the clitoris and the labia (lips) (see SexEd.net’s Clitoris Topic). The main distinction from the penis (and the vulva) of course is that the vagina is inside the body.
Like the penis, the vagina expands and may double in size when aroused, and just like there are many different penis sizes (see SexEd.net’s Penis Size Topic), there are variations in sizes, lengths and looks among vaginas.
The Kamasutra by Vatsyayana (an ancient Hindu document on sex and pleasure; other spellings include Kama Sutra) has classifications for penis and vagina sizes and ranks them by compatibility.
Some medical professionals say that foreplay and lube can assist with a vagina that seems “too small” or “tight” for a “large” penis (see SexEd.net’s Foreplay and Lubrication (Lube) Topics), but there may be other reasons the penis/vagina may cause pain or discomfort during penetration.
“Tight” and “loose” are some of the terms some use to describe how a penis feels inside a vagina, either snug (tight) or very little sensation (loose). Some men feel that the vagina may get “loose” after childbirth or some women are slut-shamed (see SexEd.net’s Slut-Shaming Topic) as having “loose” vaginas from too much sex. Just as there are stereotypes portraying an entire race as having the longest or smallest penis, some men think some women have “tighter” vaginas.
Childbirth and age are some factors that may alter the appearance and other aspects of the female genitalia. According to medical professionals, the vagina is elastic even after vaginal childbirth, and some exercises can strengthen “loose” vaginal muscles.