I'm no evolutionary biologist so if you have actual knowledge you can tell me I'm a moron.

I noticed how huge some extinct species were. There was an otter-like creature that was the size of a modern lion, an armadillo what looked to be as big as a large crocodile, and all sorts of beaked creatures that are as big as houses. What happened to the giant species?

I wonder if evolution is similar to computer evolution. Early computers were massive. Some took up entire rooms. As technology has advanced, computers have gotten increasingly smaller, while processing power has increased exponentially. Maybe evolution has a similar effect on organisms. The competition over food resulted in a size arms race. Eventually, though, the margins diminished to the point that size increasing was no longer worth it so they had to evolve in other ways. That's when organ systems started to become more efficient, allowing organisms to decrease in size while maintaining, or increasing their capacity for processing nutrients.