We got some feedback recently from an OC on 1-08. Generally speaking, the big guys had less trouble on dismounted ops (not surprisingly). The "gazelles" - AKA 20-minute-5KM-guys had it far rougher as the combat load's quite heavy and humping over grape fields/walls/etc. demands quite a bit when done for prolonged periods.
Overall, the "big guys" (assuming they were of comparable fitness levels) had it quite a bit easier than the smaller guys. "Big" generally describing someone 185/190 - 225. Being 6'2, 185lbs, I'd be at the lower end of the "ideal" bodyweight/carry-weight ratio.
Unless they're a fitness beast, a 120 lb person is going to have a much harder time carting 80 lbs of water, ammo, etc. than someone 200 lbs and it's not as though the combat load scales to your body weight - everyone's carting roughly the same amount (save C6/M203/signallers). Bigger muscles carry more weight - it's basic physiology. If you have a 175 lb guy and a 225 lb guy do deadlifts (assuming equal fitness levels and expertise), the 225 lb guy is far more likely to lift more in absolute terms.