I feel that there must be some physiological stressors or environmental factors that inhibit modern organisms of all domains from reaching the size, of say, a megaladon or brontosaurus.
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Climate stability is what allowed them to grow so large. There was a very stable climate for many millions of years, which allowed species to become hyper developed for those environments. When climate changes, the smaller species with a higher generation rate consistently fare better than larger species, often due to food scarcity and rate of change. The same thing happened in the past 15 thousand years – rapid climate change from an ice age caused most of the megafauna to go extinct.
The issue is that large species usually need an abundance of food, which requires a stable ecosystem. If 80% of the plants in an ecosystem die, all the larger herbivores and predators are going to die off, but the small herbivores will likely survive, as will their predators.
This sort of thing can be seen in real time by watching the arctic circle. When the flora freezes over and food becomes scarce, all the large fauna go away. The polar bears hibernate, the wolves and caribou move south, etc. But the foxes and the hares stick around – the hares can survive on the scarce flora, which supports a small population of foxes. Once it warms up and the flora comes back, so do the big fauna.
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