I'd say that if you want to run well in the muskeg then you need something other than stock tires no matter what machine you get/have. Most (all?) UTVs run the same size tires on their 500's as their 900+ machines so a bigger machine only means more HP and speed. With crappy tires on a bigger machine you'll just get stuck deeper, faster. A lot of the folks that run in the deep mud/gumbo/muskeg run big tires. Big tires are heavy so more HP to the ground is good and higher performance clutching helps get it there. Mandatory? No. Just better.
You should really stop by a Polaris dealer and check out the new XP 900. It's a whole new beast with the Rzr 900 motor but tuned more for torque instead of top end.
I say check the UTV specific forums and talk with folks running the machines you're interested in that are in the type of terrain you are running in.