You're looking at a lot of procedural data where for example the patterns behind the sprites can change without having to create a new sprite. Where the palettes were separate from the image data so they could look different without having to use double, triple the storage. These things can't be picked up on by png, it's not a smart enough format to be able to arbitrarily detect all repeated patterns of varying sizes and turn them into what amounts to a procedural level script.

I can't find a good exhaustive sprite sheet of SMB (this is the best I can find, but it contains a ton of repeats: https://www.mariouniverse.com/sprites-nes-smb/). I'd love a visualization of just how tiny SMB is in terms of what's stored in it.