Wapiti, Moose are just like Elk, in that there will be really big bulls, mid size bulls, and somewhat smaller bulls. I can't speak for the Shiras Moose, but for Alaskan Moose, I've seen rear quarters that are in the 140-150 lb range. I wouldn't try humping it myself as once you screw up your back, it's never the same. The bull I harvested this year I would call a mid size bull) had a hanging weight of around 500lbs, with the rears at about 100lbs or so. We left the ribs at the kill site and took the meat strips instead, since we were doing a float hunt. This reminds me that I have a story to post on this years float hunt too. I have humped large Elk rears and would guess that the Shiras Moose rears would be somewhat larger. I would recommend you ask the biologists in the area you are looking to hunt to get a better idea of size. I'm sure Fullcry, as a guide can give you better numbers than most of us can.
Take care,
Garion