Wow that’s impressive.
Is it because the first two were prototypes so they bolted on whatever they needed without care for it looking nice or did they just refine it so much that they were able to simplify the design significantly?
That and the march of progress, despite what some people believe we are still evolving at a rapid pace and engine outputs have significantly increased recently.
The first two prototypes looked like what traditional rocket engine looks like.
For example:
The RS-25 engine that equipped the space shuttle and now the SLS launcher
The Vulcain 2.1 that is on the Ariane 6
SpaceX put a lot of work on refining the engine, they also already produced more than 600 engines ! It's a crazy amount !
It's more of a refinement thing, where anything not absolutely necessary is removed, or 3D printed straight into the structure of the thing instead of added to the side.
I’m sure there is also a lot of diagnostic equipment that became superfluous or redundant once they figured out how the engine ran.