The muscle doesn't matter, what matter if your surface/volume ratio, because you lose heat as a function of your surface, and generate heat by volume. Two more things:  The shape that's giving the smallest S/V ratio, is the sphere,  The bigger something is, the smaller the S/V ratio. (1cm^3 is S/V=6, but a 2x2x2 cube is 8cm^3 and S/V=24/8=3)  And that's the reason why mammals don't go smaller than shrews, that colibris spend their time eating, and that animals close to the poles tend to be bigger, and rounder.