system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: why were dinosaurs so big?
gpt
First of all, it's kind of a bias. There were many small dinosaurs but the big ones were really impressive and that makes them memorable. Dinosaurs were on Earth for about 200 million years. That's an incredibly long time and it allows for a lot of species to rise and fall. 

We think of the era of big dinosaurs without even realising that more time separates stegosaurus and tyrannosaurus and than tyrannosaurus and humans.

And we have plenty of large animals today. The blue whale is the largest animal that ever lived. And biologists define megafauna as any animal that is at least 40kg. The overwhelming majority of all life is microscopic or at least very small, just like it was in the age of dinosaurs. But we still have plenty of animals that count as megafauna.

Anyways, species grow large when the advantages outweigh the disadvantages. And that can happen for various reasons.

One of the big advantages of being large is safety from predators. Another is being able to store a lot of energy reserves in a large body. Big bodies are also very energy efficient in extreme cold. A tiny animal like a mouse is almost all surface area losing body heat. But a large animal has much more volume compared to its surface area and retains heat better.

The big disadvantages are that you need a lot more energy to be that large. That means requiring more food. And you can't hide as easily from predators.

Take the blue whale for instance. This is the largest animal that ever lived and its an extreme specialist. Very few places can supply a blue whale with enough energy for that enormous body. That's why it travels around the world for krill blooms. These are annual events where microscopic creatures called krill reproduce in enormous numbers.

During these krill blooms, blue whales eat enormous amounts of krill in the arctic seas to pack on enough fat for the energy reserves to travel around the world. Their size protects them from the cold water. Their size allows them to pack on enough fat to survive without eating until the next krill bloom. And their size makes them fast enough to make these long journeys in time.

Because their journeys will take them from the cold arctic seas to warmer waters to have their babies. And then back again to the arctic for the next krill bloom while they nurse their babies nice and fat to survive the cold.

A smaller animal could not have managed this extreme lifestyle.

The cold in general also creates a phenomenon called arctic gigantism. As mentioned, larger bodies simply retain more body heat in extremely cold environments. Many arctic species grow larger than their relatives in warmer climates. From the crustaceans on the sea floor to the bears and seals on the ice.

Island gigantism is a phenomenon where lack of evolutionary pressure causes size drift. Under most circumstances, being big really isn't much of an advantage. You're more easily spotted by predators, you need more food to survive, and you have a harder time hiding or burrowing. Simply put, there's evolutionary pressure against growing too big.

But on some isolated islands that are devoid of predators, species lack this evolutionary pressure. They slowly evolved to be bigger than their mainland kin. Not because it's an advantage but because nothing is stopping them. There's no lack of food preventing them from growing big. No predator that eats the individuals that fail to hide etc.

Many large dinosaurs lived during periods in Earth's history where the Earth was extremely fertile. Warm and humid, the Earth was practically a greenhouse.

This situation removed the main obstacle to growing really big by providing abundant food for herbivores to grow big on. And with prey animals growing big, predators inevitably followed suit.

But it's important to remember our biases. There were a great many smaller dinosaurs. We've found dinosaurs the size of a hummingbird. And we've found plenty of dinosaurs the size of chickens, cats, sheep, horses etc. Still megafauna but not what you're thinking of.

Not to mention that dinosaurs weren't alone in their world. There was plenty of life of an unremarkable size that simply wasn't a dinosaur.