While true, this isn't the right way to analyze this. The right question is how much strength or size is necessary to operate a modern powerful chainsaw with a sharp chain. The answer is "not much". You need to be able to position a ~15 lb (7 kg) saw at waist height for a few seconds until each of three cuts is started. With a sharp saw, the forces are negligible after that for 2 of the 3 cuts required. With good technique, you can even use leverage against the trunk for the one short upward cut. There is considerable skill involved, but brute strength is not a requirement. While you are presuming a male 16-year-old here, I wouldn't be surprised if most female 14-year-olds were physically strong enough.