Why do cimpanzees have much higher strength per kg?

[Angilion]

Chimps have been seen lifting over 4 times their own bodyweight, apparently without much effort. There's a recorded incident of a chimp weighing 135lbs registering a one-handed pull of 1260 pounds on a dynamometer, which is in the region of 10 times what a human of that weight could do.

How do they do it? It's not like chimps are massively muscular.

It isn't just chimps, but I had some numbers for them and they're pretty close to human in terms of mass.

 

[Angilion]

My question was open to two interpretations, which I didn't realise. I was interested in the physical differences, not in the different evolution that led to them. It can't be as simple as living conditions, because a 10st human is not going to match a 10st chimp for strength even if the human lives like a chimpanzee.