Arm wrestling?

Question: About strength.... I have been told by people that I have big arms... I am assuming forearms, since I don't go sleeveless. I guess I get it from playing tennis. However, when me and my girlfriend arm wrestle, I have trouble beating her when she is really trying. Sometimes , she even gets an advantage, but she has never beaten me. The only time I win is when she just gives up. She is inexperienced at arm wrestling, while I have been doing it for fun for years. How come I have trouble with her? She doesn't workout, but is overweight. We arm wrestle on the floor, so I wouldn't think she uses her weight. Is she a little stronger than me?