It's perfectly normal. Like most things in nature there is no perfect match when things are in pairs.
Women's boobs are very rarely a perfect match, same with testicles, eyes, hands... everything. Sometimes it's more obvious that they aren't a perfect match, sometimes it isn't visible at all.
Regardless, it's far more obvious to you than it ever will be to anyone else. Don't worry, it's perfectly normal and there's nothing you've done (or not done) that could have caused it or could change it.
My left boob is bigger than my right. I'm seriously the only one who notices. My husband can't even tell that they're different, even though it's very obvious to me.