I'm sure I could research it but can you tell me why and how the dinosaurs got so big?

It was just that species, just like elephants are big, same as whales.

What I’ve read a long time ago, was the higher oxygen percentage in the atmosphere during the cambrian explosion.

This is why we also had insects the size of dogs.

Probably more abundant food and resources as well, because humans weren’t here to kill and consume every resource.

And if the dinosaurs really are the ancestors of birds, their weight may not have been accurately represented by their size. Remember, the fossilized bones we find have no organic material left, only minerals, so the bones may have been much lighter weight, which might encourage greater growth.
Totally an uneducated guess, but really, Andean condors can fly, despite their size.

@canidmajor You are smarter than the average bear!

“Dinosaurs’ air-filled bones were lighter than solid bones, but still strong. Sauropods’ lighter neck bones were another reason their necks could be so long.”

From the UK Natural History Museum

Here’s a good article about it.

I’m guessing optimum growing conditions for both the species and its food supply. Herbivores, that is. Obviously herbivores were the food for the carnivores. If you take most animals and give them an environment in which to allow growth, they will grow larger than normal.

I once had a pond and I killed it. I was trying to get rid of algae and ended up driving out all the O2 in the water and killed all the fish. When I thought it had finally recovered (still had no fish) I went to the pet store and bought something like 10 feeder goldfish. I was going to toss them in to ensure the water was recovered. In an amazingly short period of time they were all the size of big carp and were breeding like crazy. But the fish were given a space and food that allowed them to grow bigger than the normal goldfish and they did.

Good question.

My impression was that there was an evolutionary arms race for some many millions of years. Big flora, big fauna. Bigger meant more access to resources, and more protection from predation/more ability to predate. This all worked fine until the climate took a turn for the colder, and resources started becoming scarce, at which point being big became more of a liability for most species.