[DesuMaiden]

This article explains why.

http://www.apeculture.com/movies/godzilla.htm

Gravity makes it highly improbable that a 100 meter tall lizard could possibly exist. Godzilla weighs somewhere around 60,000 tons if you scale up a regular-sized lizard to 100-meters tall. There is no way an animal that size could possibly exist. Its because of this thing called the square cube law.

If you have a 1 cm wide, tall and thick cube and double its size, it will become twice as tall, wide and thick. Therefore its volume will increase by 8-fold, and its weight, which is proportionate to volume, will also increase 8 fold assuming the density remains the same. But the cross-sectional surface area will only increase by 2 by 2 or 4 folds. And the cross-sectional surface area is what determines the structural strength of an object. The volume and weight of an object increases by the power of three as an object is scaled up, whereas the cross-sectional surface area (which determines how much weight can be supported) only increases by the power of two as an object is scaled up. This means the mass/volume of an object grows much faster than cross-sectional surface area of an object. In other words, an object's mass increases much faster than the object's ability to support itself.

This means as an object becomes bigger it will have a harder time just supporting itself. This has enormous implications in biology. In fact, this law is called the square-cube law, and it is one of the most important laws in physics. The square-cube law is perhaps the most important law in biology because it limits how big or small an animal can be. It also determines the size of an animal will have profound effects on the physiology of an animal.

Here's another article that explains the implications of the square cube law on biology.

http://fathom.lib.uchicago.edu/2/21701757/

As an object becomes bigger, the harder it falls. In other words, as an object becomes bigger, the greater impact a fall will have it. For example, if you drop a mouse off of a 18 story building, it will receive a slight shock and then walk away; a rat is killed, a human is broken and a horse literally splatters.

I don't need to explain this in anymore detail. Physics ultimately determine the biological make up of animals. And like I mentioned, if you don't understand the square-cube law, you don't truly understand physics, biology or science in general because the law is absolutely fundamental to science. By the way, the square-cube law was first noted by Galileo in 1638, so it isn't some new, breakthrough scientific discovery.

Anyways, I hope this enlightens you. Physics is probably the most fundamental science there is because all other sciences are subservient to it. I believe physics is the most interesting science.