The best answer comes down to two factors; oxygen and food. Oxygen levels were much higher 45 million years ago than they are today, and exposure to high amounts of oxygen have shown to drastically increase the productivity of the human body, so why not the same for animals. Second comes down to food. The more food, the more of an animal. The bigger the prey, the bigger the preditor needs to be or they need to be in greater numbers. When creatures like brachiosaure exist, it would understandable be difficult to kill it when small, thus animals that were larger would have better chances, or conversely animals that worked together. Of course, this is somewhat speculation and there could be other factors involved, but this is the simplest answer that makes sense.
The best answer comes down to two factors; oxygen and food. Oxygen levels were much higher 45 million years ago than they are today, and exposure to high amounts of oxygen have shown to drastically increase the productivity of the human body, so why not the same for animals. Second comes down to food. The more food, the more of an animal. The bigger the prey, the bigger the preditor needs to be or they need to be in greater numbers. When creatures like brachiosaure exist, it would understandable be difficult to kill it when small, thus animals that were larger would have better chances, or conversely animals that worked together. Of course, this is somewhat speculation and there could be other factors involved, but this is the simplest answer that makes sense.