Can I ask: Why is he so skinny?

Figuarts proportions were a bit different when they started.

Ain't that the truth. But in hand, he has some bulk. Stand him up against some of the newer releases like the Faiz mold or the Ryuki mold (which I think is the current gold standard in Figuarts right now) and he actually holds up pretty well.

The early Kabuto riders...now those were pretty strange.

Black also felt a little too skinny.

Faiz? Dude Faiz has crazy bulk :lol All of the Faiz molds have massive thighs that you could bend steel girders with. The Kabuto riders all feel strange though don't they? It's the legs and how they bend out slightly. They feel weird and boring from the belt south. I'd also have to agree that the Ryuki mold is a damn near perfect example of where the engineering should be.