I believe the aching hands is described as wrist pump. I never get arm pump, however, when riding tight trails, my wrists would pump up terrible, so bad I could hardly pull the clutch or twist the throttle. I found that if I stopped and took a rest, and let the wrist pump go away, then started riding again, it never came back all day. I discovered the death grip was my problem. Once I just started to relax and sort of let the bike go, I didn't have any more trouble. Try relaxing your grip a little, see what happens. Throttle or clutch cables with stiff pull can be a culprit, too. Some people have mentioned changing around your bars and controls, that is a very good idea too.
Don't worry...your bike isn't too heavy. I've ridden a lot of XR400's and they are great in the tight stuff. BTW...see what I ride in the woods...its not too heavy, either......;)