550lbs is pretty light for something so tall.
The cube root principle would have him weighing in at about 120lbs if it were 6’0 tall.
It must have been heavier than that.
It was all hair.......................
Your math is correct, but your mechanics, perhaps deficient. As creatures grow larger in stature, weight increases disproportionately. Leg bones area would only grow as the square of height, so their legs would have to be thicker, just as an elephant’s legs are disproportionately thicker than a reindeer’s, even more so with a spider. I suppose they must have been quite spindly more like Kareem Abdul Jabbar than Refrigerator Perry.