You are NOT over weight. I had the same problem as you. I'm about the same height, same age and same weight too. And i know what I look bigger and weight more not because I'm fat. It's because of the muscle.
If your worried about the way you look and you feel bulky, try yoga or palates. I know that sounds super boring but apparently thats the way to go if you want to be slimmer all around.
Basically, to straight forwardly answer your question, Your probably just doing a lot of muscle work. I hope I helped you!
And remember: If you have more muscle than the other girl, that just means you can kick butt so much more. :P