I understand. Toning is about bf%. If you’re looking at abs or forearm definition, it might be a byproduct of their genetics or overall training, but nobody is going to say “if you wanna throw harder, lean out.” Rather, definition will be a byproduct of an appropriate training regimen. But I’ve also read folks like Pavel claim that too low a bf% can be deleterious for athletes, and I imagine there’s a reason why Brady and others never walked around with a 6-pack despite all world diets and training.
Again, look at all the big arms people have mentioned here - Manning, Stafford, Brady. These are guys with average physiques. What’s making their arms so live is stuff you can’t tell from a shirtless selfie - or at least not in the way you’re describing. It’s muscle fiber type and technique - and yes, I believe, muscle insertion points.
Of course, there’s more than one way to skin a cat. Clemens wasn’t ever lean, and like Tyson, he had short muscle insertions but generated so much power with his legs that he found another avenue. But, look at Tyson’s jab - it always sucked because of his muscle insertions and because - I’d guess - he wasn’t super fast twitch so much as insanely powerful with great leverages and the ability to quickly throw his whole body into his punches.
For every Ellsbury, there are a million Gabe Kaplers. Looking at a physique - unless you have a highly trained eye and know exactly what you’re looking for, is a poor way to judge explosiveness or arm strength.