What's it with all the weak arms? Seriously, my arm or wrist have never been tired my any mouse weight. I prefer light mice as well, simply because they feel better, but I never had muscle problems. And I don't really work out much other than carrying my son around or some pushups now and then.
I think the problem is more ergonomical, like a wrong position of the upper body or arm relative to the desk. It must be something like this because I can't imagine anyone being that weak that some 100g mice really tire your arms (as long as you play games that allow for some rest, if you move like crazy all the time of course the arm gets tired, but that's regardless of any extra weight).