“Thus a 12-foot-tall human’s weight would be 8 times that of a 6-foot human having the same proportions.”
I sure don’t weigh 8 times what a child half my height weighs. Not even four.
Are your proportions exactly the same as that child’s? Probably not.
Biophysics wise or scaling law wise I agree with amorphous in general. This is why elephants have thick legs and spiders skinny ones: the weight goes as the cube and the cross sectional area as the square so the weight/cross section, or stress, goes up linearly, given the same proportions.
I wondered about your interesting personal observation, steve86, and found a age-height-weight graph for boys.
https://www.cdc.gov/growthcharts/data/set1clinical/cj41l021.pdf
If you are six feet tall, a boy half your height ( 36 inches tall ) would on average be 2 1/2 years old and weigh 30 pounds. You would have to be unusually heavy, 240 lbs, to be eight times as heavy, so steve86, you make a good point.
A man would have to be an unusually light 120 pound six-footer to be only four times as heavy though.