Huge dinosaurs rove mid-Jurassic Earth. Bigger is not necessarily better: larger life forms require more space and food, and have fewer offspring and fewer survival options in times of change. Microscopic "water bears," which survive today, appeared 200 million years before the huge dinosaurs. The Californian water bear loves to cling to moss and lichens with its tiny claws.
The dinosaur and the water bear are archetypes of animal mega-micro waverings. Getting smaller is not uncommon in evolution. The fossil record suggests that beings which "miniaturize" are those most likely to survive mass extinction crises.