One of the reasons that we can easily walk upright and on 2 legs is that our center of mass is located a bit above the hips, around your stomach, and not much higher. Try balancing yourself by laying on a bed and sliding out over the edge head first. When you get so far that you just start to tip, the edge of the bed will be approximately at the center of mass of your body. If the center of mass is close to your hips, this means, very roughly, that almost half your body's mass is in the legs. If you think about it, that makes some sense. Our legs are long and covered with thick muscles.
As for the arms, they aren't nearly as massive as the legs since the bones are slightly more delicate and the muscles aren't as strong.
You also have to realize that this answer will vary from person to person. Some people, like rock climbers, have very massive upper bodies and arms and more slender legs, so the way mass is distributed would be a bit different for them.
(republished on 07/11/06)