1. Masculinity is not only muscles. It implies a complex of things that are typical for males. And I am talking about the basic traits that the majority of girls want, of course, everybody has different preferences, some like fat guys, some like skinny, but these are rather exceptions.
2. There may be a lot of reasons she likes you, but either she is very untypical kind of girls or you have something she found really catchy. Lol, I even know a pretty girl who dates a fat stinky redneck because he's got a long dick