|The picture shows the relative sizes of various large animals
throughout time. Photo source: Dott & Prothero, 1994.
Large body size evolves for a reason. It can be a survival adaptation to climate change, food type and availability, increase in prey size, increase in atmospheric oxygen levels, etc. Extreme body sizes, may they be very small or very large, can be favoured by natural selection for a period of time, but as the environment around an organism changes and evolves, its body size may become disadvantageous and natural selection will take its toll to make way for a more suitable descendant. Large animals seem to only be successful temporarily relative to smaller taxa. Bacteria, on the other hand, have been successful since the origin of life 3.8 billion years ago, and they still thrive today. There are so many biological criteria that must be met for a large animal to survive and if even one is missing, the animal is doomed.
The giants are more devastated by extinction events because they have such slow reproductive rates that they cannot replenish their populations as quickly as the smaller animals can. Disease also wipes them out for the same reason. Their slow reproduction rates limit the genetic variation in their populations so that when one animal dies, the rest are likely to soon follow. They are more vulnerable to food shortages because they have such huge food mass requirements. Finally, they are simply too large to find shelter against extremely severe conditions of any kind.