Don't talk daft, tall people's hearts/lungs limbs etc are in proportion to their size/weight ... (they better fecking be, or I'm *really* going to panic about them palpitations ... )

It's sometimes known as the cube:square law.

Imagine a cube 1 metre along each side.

- Each side is 1 metre long.
- Each face is 1 square metre.
- The volume is 1 cubic metre.

Now double the dimensions.

- Each side becomes 2 metres long: twice as long as the first cube.
- Each face becomes 4 square metres (2x2): four times as big as the first cube.
- The volume becomes 8 cubic metres (2x2x2): eight times as much as the first cube.

So, if you double the linear dimensions:

- You quadruple the cross section, and
- You multiply the volume by eight.

Now, apply this to a person who is 6 feet tall and a person of roughly the same shape who is 12 feet tall:

- The taller person's linear dimensions (height, length of his legs or arms) are twice as great.
- The taller person's cross sections are four times as great. Therefore, there is 4 times as much cross section on the bones.
- The taller person's volume (and therefore his mass/weight) is eight times as much.

So, make a man twice as tall and you make his bones 4 times as strong but, crucially, they have 8 times as much weight to carry. Similarly, his muscles are strong in proportion to their cross section so are 4x as strong but have 8x as much work to do.

Thus, a giant human simply could not survive. His muscles would be feeble in comparison to his weight, and his bones would be fragile in proportion to the weight they had to carry.

People such as Robert Pershing Wadlow often need assistance to walk.

Yes, diet is an issue to, but that is part of the same cube:square:linear thing. Twice as tall, eight times as massive, 8 times the food requirement.

A "real" giant, as encountered in fairy stories, would have to be a substantially different shape from a human: squatter in proportion, stout strong legs and a proportionately lighter upper body.