Animal Size versus Strength and Cooling
A simple theory that may have already been explored by others. But it's common knowledge in physics that when an object gets bigger; weight and volume increase by the power of three, whereas surface area and cross-section area increase only by a power of two. This would explain why larger animals need to live in water, and also why smaller animals have such incredible structural strength but can't survive the cold. Since bone cross-sections and skin area increase only by 2^, they would not be sufficient to support to the expoentially increased weight and heat generation from the 3^ volume.