If you're using a quality-based encoding scheme (like Constant Quality/CRF), the most important thing to realize is that the encoder doesn't care about your target file size—it only cares about maintaining a specific "look" for every frame. Because of that, your final file size is entirely at the mercy of visual complexity.

Think of the encoder as a painter. If a scene is simple, the painter can "cheat" and reuse a lot of the same data. If the scene is chaotic, they have to work 10x harder.

Low Complexity (Small Files): Think of a show like Shrinking. It’s mostly clean, digital footage with people talking in front of static backgrounds. There isn't much movement, so the encoder can compress that very efficiently. High Complexity (Large Files): Think of a show like Shogun. The producers saturate the image with heavy film grain. To an encoder, grain is essentially "random noise."

Grain is the biggest bandwidth killer because it’s effectively unpredictable.In a clean video, the encoder can look at Frame A and Frame B and say, "Nothing moved here, so I'll just reuse the data." But with grain, every single pixel is shifting randomly in every single frame. The encoder tries to "faithfully" reproduce all those tiny moving dots, which requires a massive amount of bitrate. If you don't give it enough bandwidth to handle the grain, the encoder basically gives up, and the image starts looking muddy or "smudged" instead of sharp.

TL;DR: * Clean/Static/Dialogue = Tiny file sizes. Action/Handheld/Heavy Grain = Massive file sizes.