I ran a G2 once, heavy prime, 1000-foot mag and a matte box on a Film Masters. The arm could not handle the weight. I wrapped bunjee cords tightly around each arm section and this helped take a bit more weight but I was still lifting. The rig was around 95 pounds, all told. It was a two minunte shot, fortunately a very simple one, but it started to get really uncomfortable after a minute and a half.
One funny thing I remember was the sound guy telling me he could hear me breathing in the main talent's microphone. I guess I was working pretty hard!
Oh, and for the record I'm not 7' 300lbs either. 5'9" 150. :)