Same! I'm relatively skinny, but for some obscure reason I'm really, really good at arm wrestling. I love it, it's my hidden talent.
Mid laste year a bunch of American exchange students came into my pub, there must have been a good 20 of them, and saw me arm wrestling a couple of friends, who I duly beat no bother. One of these lads, named Wyatt from Kansas, saw this and challenged me - he was evidently used to winning. He sat down looking very smug, his arms and hands about double the size of mine. So we sit, tense up... and he pushes. The look on his face was priceless; I swear I almost burst out laughing, there was no way he expected that to happen! So we sat for a little over 12 minutes with him pushing and jerking my arm about in an attempt to get me down. Eventually my arm from the elbow on just went completely numb, and I dropped. But dear Lord did that get a round of applause: some skinny English kid nearly beating a big, burly Yank.