but hymen can also rupture after vigorous cycling and heavy exercising so thats not a good way to check virginity and thus this means if hymen gets ruptured after cycling then also vagina increases in size?
It definitely changes. You have skin called the heimen that surrounds the vaginal opening. Once penatrated, this skin opens up and the vaginal open stretches to fit the size of whatever is going in it. This is how a doctor can tell whether or not you are a virgin. Everyone is different, but usually the more people who have sex with, the more it stretches. I have heard it stretches tremendously once having children, but until then, it will keep stretching little by little after you lose your virginity.