Hey, thank you!
I sculpted the model using Arnold as the reference, then use blend shapes (also called shape keys). Blend shapes basically let you use a slider to go between two types, e.g. fat and not fat.
Then on top of that I scale the bones in the rig.
This is maybe is too much tech speak if you’re unfamiliar.
The simple version is: I change the shapes of his body using sliders like in Skyrim, then scale each part independently.