The hugest muscles are actually results of steroids or people who really bodybuilder for show. So, it helps to study body builders.
And of course since your esstially drawing muscles anyway, why not go to the shape directly. It make sense since all the skin is doing is like an elastic wrapping around the muscles, and is what gives the outside it's shape.
Just reference all the parts and shapes muscles, and then exaggerate those shapes. Then you get a better and more detailed muscle. If notice, sometimes you see even the strands of the muscle (like between the pecs) if the body is really tight and strong.
The boring truth is you need to study. Look up diagrams, look at photos, get to know how these muscle groups are shaped. From there it's a matter of playing with proportions to obtain what you're looking for.
Takes years to learn this stuff, so you better start now.
I think what would probably help the most is researching and learning to draw muscles as they are in real life and then once you become comfortable with that, begin to experiment with exaggerating them.
Posemaniacs also has "bodybuilder"-type references for ultra-muscled figures. [link] Kill two birds with one stone.