Think about the IRS, two A-arms are probably about the weight of a swing arm, you have twice the shocks, plus where the chain and sprocket are, that whole assembly probably weighs as much as a swing arm. Plus the frame goes back farther on the XX so you can mount everything to. Plus as Rkangel said, the center drive probably adds some.
2008 700xx; Power Bomb header with power core 4 exhaust, Power Programmer III, pro armor nerfs, prm products control front and rear bumpers and full skid, 10" ITP rears, ITP Baja's fronts, RZR 2's all the way around, Uni filter, 13t sprocket, CCP steering Damper, DS450 shock swap, Dual Gasser Throttle, bar risers and Moose barkBusters. 730XX kit plus other engine mods!