[whswhs]

In the words of The Science of Life, by H.G. Wells, Julian Huxley, and G.P. Wells,

"A mouse can be dropped down a mine-shaft, however deep, and arrive at the bottom dazed but unhurt. A cat or a dog will be killed, however. A man will be not only killed but smashed; and if a pit pony happens to fall over, the speed at the bottom is so terrific that nothing is left but a few of the hardest bits of its bones and a splash on the walls."

In the case of falls, this is a function of terminal velocity, which depends on air resistance. But with being hit by a truck, the relative velocity doesn't depend on the size of the man or animal.