soft question - Why is there a size limitation on animals?

Why is there a size limitation on human/animal growth? Assuming the technology exists for man to grow to 200 feet high, it's pretty much a given that the stress on the skeletal structure and joints wouldn't be possible to support the mass or move...but WHY is this? if our current skeletal structures and joints can support our weight as is, wouldn't a much larger skeletal structure do the same assuming it's growing in proportion with the rest of the body? And why wouldn't a giant person be able to move like normal sized humans do? (I'm honestly thinking Ant Man, or even the non-biological sense of mechs/gundams/jaegers)...I'm just having a hard time grasping why if it were possible to grow to gigantic sizes or create giant robots, why it then wouldn't be possible for them to move.

Answer

The following fact lies at the heart of this and many similar issues with sizes of things: Not all physical quantities scale with the same power of linear size.

Some quantities, like mass, go as the cube of your scaling - double every dimension of an animal, and it will weigh eight times as much. Other quantities only go as the square of the scaling. Examples of this latter category include

• Muscle strength (a longer muscle can exert no more force than a shorter one of equal cross sectional area),


• Heart pumping ability (the heart is not solid but rather hollow, so the amount of muscle powering it goes as the surface area),


• The compression/tension that can be safely transmitted by a bone (material strength is intrinsic and independent of size, so the pressure that can be supported is constant, so the force - cross sectional area times pressure - that can be supported goes as the square of size), and


• The ability to exchange material and heat the the environment (single cells for example have a hard time growing large because their metabolism goes as the cube of the size, but their ability to transport nutrients across their outer membranes only scales as the area of those membranes),

at least to a first approximation. You could also come up with other quantities that scale differently with size.

As a result, simply scaling up an organism will undo the balance that has been achieved for that particular size. Its muscles will likely be too weak, its bones will likely break, and it will generate so much internal heat (if it is warm blooded) that the only equilibrium achievable given its comparatively small surface area would be at a high enough temperature to denature many proteins.

For a completely non-biological example, consider the fact that airplanes cannot be made arbitrarily large, and in fact different sizes of planes have very different shapes and engineering requirements. The surface area of the wings does not scale the same way as the total mass, and the stresses and pressures the material needs to withstand will not stay constant as you enlarge the plane.