do we not read what we post?
The most obvious differences between different animals are differences of size, but for some reason the zoologists have paid singularly little attention to them. In a large textbook of zoology before me I find no indication that the eagle is larger than the sparrow, or the hippopotamus bigger than the hare, though some grudging admissions are made in the case of the mouse and the whale.
But yet it is easy to show that a hare could not be as large as a hippopotamus, or a whale as small as a herring.
For every type of animal there is a most convenient size, and a large change in size inevitably carries with it a change of form.
each body type has a maximum and minimum size (which you are correctly arguing)
but it is not to be confused with the "most convenient size"
Now that most convenient size does not happen in isolation
but occurs within the environmental conditions (weather, food availability, etc)
for an "species" to persist
its most convenient size must lie within its max/min range
But large mammals like that have a very difficult time cooling themselves. When the climate warmed up, they couldn't tolerate it. The same thing goes for elephants. They're wonderful at retaining heat but not so good at getting rid of it.
the most convenient size was changing
those already existing couldn't change their size
but possibly their offspring could
The same is true of humans. Smaller people get cold very easily--their body heat dissipates quickly. Larger people can stay warm longer and can tolerate much colder temperatures.
so during the ice age, Larger people would be favored
these days, not so much
Now, take out average 5'7" and scale that up to 8 feet, which is what we're talking about here. First off, you'd lose a lot of muscle strength. Remember, the cross section of the muscle increases by the square of the scaling factor and the mass increases by the cube of the scaling factor.What you'd have is a massive body without the musculature to support it.
Second, the cardiovascular system would have to work really hard to pump blood and oxygenate tissue, which leads to lung issues as well. Why do you think that many people 7 feet or larger die from pneumonia?
You are looking at it backwards
Now take an average 8 foot ice age human who evolved to survive those conditions and and scale him down
to 5' 7" of recent times
These giants may have been larger (the most convenient size) but still within the max/min range
so the entire body (heart, lungs, etc) evolved to handle the size
it's not like a child from a small family suddenly grows to 7 feet tall
when conditions changed
they became disadvantaged
and had to rely on the smaller humans for existence
so they became priests, shamans etc, who don't actually work