So, the only way I could try to figure this out was to compare Hirako to another character whose muscles have been confirmed.
Hirako and Uta look around the same size in the manga but according to their wiki bios, Uta is 5 cm taller than Take. And yet Take is 11 whole kg heavier than Uta.
We can safely assume that Hirako is really built (way more than Uta at least), very muscular…possibly also with a nice soft tummy. I mean, unless there’s a way to calculate muscle mass off of only height and weight, I think that’s our best guess. You know, math-wise.