The mechanical reason is because sprites had to be compact and functional.
The lore reason is upgrades.

Its because he hid himself while he rebuilt himself.

To me, people are overcomplicating Zero's redesign. It's just an art style change. No matter what the lore implies, it has to look different. Even in childhood, by the time even I saw the game's box art, I understood that. Can't blame people for digging every nook and cranny that's left of the Mega Man content that we got, though, but there's not much else of importance, either.

"Defeats his evil X and their four kids" took me a sec 😅

I'd still like an Elf Wars game in this style.

From what I understand with the lore and the series itself, it isn't just a design choice, because by ZX it was stated that since "Reploids" had upgraded so much, that the only easy way to tell them apart from a human was the crystal on their foreheads, and by that era humanity was also getting used to getting upgrades too, as some humans had been getting cybernetic upgrades so they could compete with reploids, and by the Legends era the "Carbons" couldn't be told apart from humans and reploids, so thats why it makes sense that it was just the technology getting better.

This is one of those things that would be easy to sort out definitively if there weren't so much conflicting information.  Capcom staff, Inti's staff, and Inafune all have conflicting points.

The only part of it that has been consistent is that Capcom technically considers every Mega Man series a "possible future" deal, so there's no strict canon that isn't specific to a series.

I have my own explanation, both x and zero's  x series body was badly damaged after the elf wars. Because their bodies were  a mystery to many scientists, they couldn't be rebuilt. So they gave them  new sleeker bodies to keep up with the current technology.

Phoenix magnion uses an attack that shows repliods from the X series as they appear in that series, and since they look accurate, then it would make sense that X and Zero would as well, I subscribe to the theory that during the elf wars X and Zero were modified possibly to use cyber elves.

We may forget that in the beginning we only see Zero's upper half missing the torso and Passy (Ciel's companion Elf) basically merge with it giving Passy's design to Zero. Of course that is just a theory but even X's design in Elf form is new so the Elf design might be one of the driving force for this redesign. Also, X looking like that is because he fought in the Elf War, meaning; just like how he can copy their attacks, he can copy their style of armor.

Force Metal was explicitly stated to be from outer space. I don't think Dr. Light or Wily planned for reploids to find kryptonite and use it as steroids.

I like both artstyle. I can accept the theory of the advance armor layer thingy.

Big chunky OG armor Zero is cool. Slick run N gun/slash slim Zero is cool. ZeroExE is cool. All Zeros are cool :D

Thanks for making this video! I was pretty curious about this myself.

Honestly, Omega’s appearance in the final battle has always seemed rather straightforward to me. We see him altering his own body (Zero’s original body) multiple times with the Dark Elf’s power. He keeps going bigger and bulkier, but Zero manages to keep up with and defeat him every time. So, Omega just says “screw it” and changes his body again to mimic Zero’s current one: sleeker, more agile, yet still retaining some of Zero’s abilities from the X games.

I think it's worth mentioning that a MMZ era design for reploids are more human looking, which fits with the series trend/theme of robots and humans becoming more indistinguishable as the timeline progresses. So it's always made sense to me that Zero's new design is meant to be an upgrade in-universe. When I first saw Omega, I did think it would have made more sense for him to look like MMX era Zero, but I figured that the developers probably just didn't want to create a new version of that sprite, so I just sort of pretended that he did have the old design in-universe.

I subscribe to the idea that as tech got better. Zeros body got smaller, sleeker.

I personally like to think that the body zero inhabits in this series is just a standard reploid body made for non combative reasons because this version of zero does not have a buster or any weapon attached to his body (until the z knuckle), which is why he uses a buster shot rather than a buster.

I find this interesting that after so many years people are still debating about this! Just goes to show how passionate the fans are. Did you all do the Capcom survey?

My head cannon lore about the design choice is that ReploidForce/MaverickHunters kept improvising personnel alloy for further environmental adaption.

Zeroes case was more to make him easy to integrate him in society as he was much more feared than other Mavericks.

Weirdest thing in megaman: Reploids having a cannon in each leg and still running fine, without tripping