I will very briefly tell you why animals today are much smaller than they were in the days of dinosaurs. Dinosaurs are reptiles i.e. cold-blooded. They rely on the ambient temperature to warm themselves up. This is why they live in warm climates. They require less energy to maintain body temperature than for warm-blooded animals such as the human-animal, and therefore less food to achieve this. That’s one reason.
Another reason is food resources. There are less food resources on the planet today than there were in the days of the dinosaurs. The planet was covered in forests in those days. Scotland is almost devoid of forests when a million years ago it was covered in trees. Humans cut them all down for construction – mass environmental destruction. When food resources, in general, are lower or less abundant, animals tend to shrink in size because they require less food. I guess too, their population size also reduces to accommodate the available food resources. Therefore both size and numbers diminish. The human population has grown exponentially and massively which means less space for animals and less food resources for them and indeed less habitat. They all combine to reduce the size and numbers of animals.
At the beginning of the world, as I understand it (and I am describing this out of my head without research) single celled creatures developed from bacteria and these evolved under Darwinian principles into huge dinosaurs over hundreds of millions of years, indeed billions of years. So it’s a cycle. “Animals” were very tiny initially and very crude and due to the abundance of food resources they grew into giants but there are reptiles. The dinosaurs became extinct it is believed because of an impact by a meteorite in the Gulf of Mexico. The replacement were warm-blooded animals which never could grow to the same size and which continue to diminish in size and will do in the future as will their numbers.
Taking one species, the climate across the planet affects the size of the species. The Siberian tiger is bigger than the Bengal tiger. The bigger size makes an existence in a cold climate easier, which is why the Siberian tiger is the biggest tiger species.