One differance, the oxygen content of the atmosphere was higher than the present 20 0dd percent. There were, for instance, dragonflies the size of small dogs. Not possible with our level of O2.
But the idea of a fight between Bront and Rex, even if they lived together, I don't think Rex would even challange such a brute. It is easily twice the size of even Rex and one sweep of that giant neck and Rex would be in the next county.
The post that was quoted here has been removedWhen a species has little competition it tends to grow larger.
When Insects ruled the earth we had massive dragonflies
When Amphibhians ruled, the frogs were huge
Reptiles: dinosaurs.
Reptiles could become even bigger because of their superior bone structures and blood systems.
By the time mammals came along their was much competition and a bigger rate of ecological change, this tends to stop making very large organisms evolve as they have slow reproductive cycles.
However some mammoths and sloaths were huge, even sabre tooths were massive compared with the biggest cats today. Unfortunately many of these big animals were easy prey to humans and have disappeared in the last 2 million years.
The Blue Whale is still the largest animal ever found.
Also remember the dinosaurs were aroud for a very long time and we tend to cherry pick the big ones, some of which had a short span.