1. Gravity: The force of gravity pulls down on animals, making it more difficult for them to support their own weight as they get larger. This is why very large animals tend to have thick, strong bones and muscles.
2. Oxygen requirements: Animals need oxygen to breathe, and the larger an animal is, the more oxygen it needs. This is because the surface area of an animal's lungs does not increase as quickly as its volume as it gets larger. This means that very large animals have to breathe more rapidly than smaller animals in order to get enough oxygen.
3. Food requirements: The larger an animal is, the more food it needs to eat. This is because very large animals have a high metabolic rate, meaning that they burn through energy quickly. This means that very large animals have to spend a lot of time eating in order to maintain their weight.
4. Thermoregulation: Animals need to be able to regulate their body temperature in order to survive. This is more difficult for very large animals because they have a large surface area relative to their volume. This means that they lose heat more quickly than smaller animals, and they have to work harder to stay warm.
The combination of these factors means that there is a limit to how big land animals can become. The largest land animals that have ever existed were the sauropods, which were a group of dinosaurs that lived during the Jurassic and Cretaceous periods. Some sauropods, such as Argentinosaurus, were estimated to be over 100 feet long and weigh over 100 tons. However, even these animals were limited in their size by the factors discussed above.