"The bigger they are, the harder they fall." You know, I think that’s generally true. Of course, I also believe that the older they are (or the older I am, anyway), the harder they fall. When I was a little kid and made accidental dismounts from the horse, it seemed like a longer distance to the ground than it does now. Despite the reduced distance vs. my height, the impacts are a great deal more solid now. I don’t check the weather report, I check the gravity report. "Things gonna be heavy today?"
Newton’s second law says that force equals mass times acceleration. That’s why when I was a tyke and supposed to be napping, I didn’t get hurt when I played "magic carpet". I’d hop off the top bunk with a pillow under me and be amazed when I didn’t die — of course, I only weighed about 20 pounds. Now (in addition to being a fragile old geezer), I weigh more, so more mass. Add more acceleration, like the time I leaped off the loading dock chasing a dog, and the force of landing was enough to break my heel. A word of advice: if you’re going to make some sort of gravity-defying leap, get a stunt-double.
This mass times acceleration thing is also important when little puppies fall off tables, or get dropped, or fall down stairs, or out of cars. Little puppies don’t weigh much, and this low mass often results in avoidance of injury even when the puppy has taken a rather appalling fall. Unfortunately, if you’re going fast enough (like in a moving automobile), you can still get enough force to be fatal.
Even when you’re going slowly, you have to consider your point of impact. It’s possible to land just right and dissipate your energy by rolling, and I’ve seen dogs that bailed out of pickup trucks at forty miles per hour get away without a scrape. I’ve also seen them climb out of the window of a parked car and break a leg on impact.
Most of the time when a kid is wagging the dog around, the distance to the ground is so short and the dog weighs so little that drops are pretty minor traumas. Sometimes, you hit "just wrong". Back in 1978, I saw a puppy that only fell 3 feet, but he hit his head on the edge of concrete step. He was in a coma before they picked him up again, and he didn’t last an hour.
Today we’re treating a one-pound puppy who got dropped and whacked his chest hard enough that it bruised his lungs inside. He was unresponsive and not moving much air when he arrived. Fortunately, with some anti-inflammatories and several hours under oxygen, he’s starting to come around a little. He’s still having some breathing problems, but you can hear the air moving in his chest now. Plus, he’s walking around his little oxygen chamber instead of just lying there like a lump.