Change happens because of puberty, not sex.
As for why girls get labiaplasty, it's due to insecurity. There's so many things that can contribute to it, and even lately I've been slightly doubting my own hoo-ha. For example, about a month ago, one of my friends on facebook posted this article about a guy who went to Russia and had determined that Russian women are the most beautiful women in the world, and made this comment about how other women have "roast beef" whereas most Russian women have very small labia, that they have the perfect vagina. As another example, if you take a look at the "who has the prettiest pussy" thread, you might notice that the majority of the girls who are mentioned are girls who have very small labia.
It's things like this that give a lot of women insecurity about something they should not feel ashamed of. It's no different than a man's insecurity about his penis size. Everyone is packing something different, it's completely normal to have either small or big lips, small or big dick, small or big boobs, small or big ass. When one is more glorified than the other, it gives people doubts about themselves.