The guy in the picture isn't crouched down and leaning into the M60, it's just as straight as he can stand with the weight.

Actually 650 rounds of M80 ball only weighs about 42.5 pounds. However, after carrying it and a 23 pound M60 all day long up and down hills, in soft sand, through mud, or heavy foliage that 42.5 pounds (plus the weight of the pack) will feel like 100 pounds.