Is there a gap in the sizes of animals?

Most animals are, from our perspective, fairly small – rodents, weasels, most birds, most lizards and most fish. Then there are animals that are large – humans, big cats, bears, deer, alligators, elephants, rhinos, ostriches and white sharks. There does not seem to be much in the middle, from about 40 pounds to 100 pounds. You can see this really clearly if you look at a single type of animal. In the cat family, there is a pretty big jump in size from bobcat to cougar.

Is this observation correct and, if so, why should it be? I am thinking that there is something similar among plants. There is a pretty big gap between the sizes of bushes and of trees.