>>2974
Depends on the version. Since that's the design from "SatAM," the Saturday morning Sonic show from 1993, which had its design used in the Archie comics, I can explain Robotnik's origin in the Archie comics. Tens of thousands of years ago, in our present, there was a nuclear war that nearly wiped out humanity. This mutated animals into the humanoid freaks like Sonic and his friends. It also mutated the remaining humans into grotesque cartoonish mutants, with four fingers on each hand, and other odd features. They weren't called "humans" anymore, they were called "Overlanders," which was later revealed to be to differentiate them from regular humans who hid in a city under a mountain to protect themselves from radiation and didn't mutate. (This was Station Square, from Sonic Adventure, to explain why those humans looked different than the ones shown in the comic previously. Though the name "Overlander" was used for like two years before Sonic Adventure, so I'm not sure if they were planning that reveal even before they knew about Sonic Adventure).
But anyway that only explains why Robotnik looks gross and cartoony. Overlanders look kind of fucked up sometimes. But Robotnik himself is still also just really fat for an overlander. Notably his brother, Colin, is a square-jawed chad, and Colin's daughter, Hope, also looks pretty normal. But there are other fucked-up looking overlanders, too, like the giant-headed genius, Nate Morgan. And Robotnik's nephew, Snivley, looks like a little midget goblin man. That pretty much covers all the important overlanders, and most of the ones in the background look pretty normal, but it is explicitly stated they're mutants and that's why they look different from the regular humans in Station Square.
Note that the above explanation may also apply to the Sonic Underground TV show. This was never explained in the show itself, but that show uses the same Robotnik design, and the show does cross over with the comic in one issue. The world of the show and comic are quite similar, though of course with notable differences, but you're probably meant to assume they're similar except for the things that are more explicitly different. Robotnik is portrayed as pretty much the same, though, except not dead. Robotnik was dead in the main universe by this point.
See, eventually Snivley killed Robotnik, but the Robotnik from an alternate timeline where he actually won that fight by turning himself into a robot didn't like the idea of a timeline that was exactly the same as his except Sonic won, so he went to that timeline (the main one) to try to kill Sonic. He then put himself in a new robot body that looked like the one from Sonic Adventure. So that one is explicitly not a real human body either. Later, some aliens called them Bem came along and had technology that turned everyone who had been turned into a robot organic (except for Sonic's father because he was the only one actually roboticized for medical reasons before Robotnik stole the tech for evil, so turning him back would have killed him. Also it didn't fix half-robot Bunnie, and I forget if they ever explained why). But my point is, that Robotnik body from Sonic Adventure onwards is technically organic eventually, but it was originally a deliberately cartoonishly designed robot that just got turned into flesh, which again explains why it doesn't look normal. He's that obese because he was originally a robot just designed that way.
Meanwhile, in the British "Sonic the Comic," Robotnik was originally the skinny Dr. Ovi Kintobor. He was doing experiments with the Chaos Emeralds when he went to the fridge to grab a snack of a hard-boiled egg. On his way back, Sonic from the future came back in time and tugged on the power cord of the Chaos Emerald machine (called the ROCC) to trip Kintobor. He crashed into the machine, with the egg, and got fused with the egg and also the evil energy of the Chaos Emeralds, and it turned him into the egg-shaped Dr. Robotnik. Some time later he succeeded at taking over the world, and then one day put himself in a giant egg to incubate and mutate into another, grosser-looking form, which resembled the one from the other 1993 TV series, Adventures of Sonic the Hedgehog. That cartoon was comedic, but in this comic this design actually marks the point when Robotnik starts acting much more dark and evil. And oh yeah, he was technically created by Sonic, but that's only because in the future, Robotnik's army of Metal Sonics, called Metallixes here, went rogue, went back in time, and took over the world. Sonic noticed that in taking over the world they made it so Kintobor never became Robotnik, so he reasoned Robotnik was necessary to stop them, so he had to create Robotnik in order to stop Robotnik's own creations. Now, if Robotnik never existed, wouldn't The Brotherhood of Metallix never exist? Uh... I guess they're just immune to paradoxes due to being on the time-travelling Miracle Planet (known as Little Planet in the games).
In all other continuities, though, including the games and the current comics, Robotnik just eats too much. I suspect this will probably apply to the movies if Jim Carrey ever just puts on a fat suit for the role. Robotnik is skinny in the movies, which is weird.