system
Given the following post, respond as if you're commenting on Reddit:
human
(Dragonball Z) Why is Goku slimmer as a Saiyan God?

That's it.
gpt
Honestly, Toriyama has been drawing all of the characters a lot skinnier recently.

If you specifically mean in universe, then maybe SSG is so fuckin crazy powerful that more muscles would make zero impact on his strength, and he doesn't need them for intimidation. Less muscles also equals way faster. (Remember super buff Trunks?)