Where you going in BC? I know a couple of guides and outfitters, usually the guide will quarter, pack, skin the game etc BUT with a big animal like an elk two people make it ALOT easier so its a nice gesture to get in there and help if your guide wants/needs it. For the backpack depending on where your going you should be able to get the horse right to the animal and load it right there, or you might have to pack the quarters out to a suitable place then a larger pack is the only way to go. having said that I really like good sized day backs, such as this, 30L, will hold lots of gear for the day, and comfortable.
Unless I am packing the animal out, then I got with a large External frame pack. This is my frame(not the best pic, but good idea of size- I'm around 5'10'') I use it exclusively in back pack hunts, on this trip it was 15 miles into the country, andaround a 100# on the pack out. Doesn't hurt the back, hips, etc.
I would suggest you ask the Outfitter, they will be able to tell you what you need/don't need.
Kirby