Why does Goku Black look so skinny?
His original appearance was the same as his counterparts. After stealing Goku's body, he underwent physical changes. While still essentially identical, he has a slightly darker skin tone and fully outlined eyes, along with a somewhat skinnier physique. In the manga, he has dark outlines around his eyes.
Why does Goku Black have Goku's body?
Even after Goku Black's backstory is summarized, it still doesn't quite explain why Goku Black turns pink. Zamasu's wish transferred his being into Goku's body, meaning he is now technically a Kai and a Saiyan, with access to all of Goku's Saiyan power.
Why is ssg so skinny?
It's a shift in animation style and mindset, in contrast to Dragon Ball Z where bulging muscles were indicative of strength; Toriyama has decided on a more refined look for SSG. Therefore the most powerful characters are not hulking masses (see : Broly).
Why did Goku lose muscle?
It looks like he is more youthful and perhaps this is why he is thinner. Toriyama has always had a nack for making the more powerful characters smaller. That or he just stopped lifting. Bottom line here is Toriyama created him that way.