The problem with Levis 501s is that they are simply not what they used to be. About 25 years ago, Levis started screwing around with the cut, place of manufacture, and the denim used. And then there were all the pre-washed/pre-shrunk/pre-softened/pre-whatevered versions... Why a company would fuck around with such a completely iconic product, I have no idea, but I'm sure some whiz kid with an MBA can explain why it was absolutely necessary.
So, if you want a real pair of 501s, you need to go to a vintage store. Unfortunately, they will already be molded to fit someone else's body (see below). I haven't bought a new pair in 20 years, but I have a closet full of vintage finds. (Of course, vintage stores aren't what they used to be, either. But such fond memories of the original WoW store in San Francisco, where you could just strip naked in front of the shelves of second-hand 501s, try them on, one after another, and get the opinion of the other customers...)
But back when there were authentic classic 501s, here was the procedure for getting a bulge and dealing with muscular build:
1) The only kind sold back then was shrink-to-fit, and they were stiff as a board when new. In those days the labels were true to unshrunken size, unlike the present values adjusted flatter people. You bought a pair labeled about 1" bigger than you waist (a bit more if muscular) and about 3 or 4 inches longer in the leg to allow for shrinkage. Trying them on was sort of pointless, since they were going to shrink all over and get stretched out in some places, but could give you some vague guidance.
2) Next, you thoroughly wet the jeans in warm water, by putting them on commando and soaking in your bathtub, for example. Warning: The blue dye will run and permanently stain everything in your bathroom, and the rest of the house as you leave the bathroom. Maybe better to use the garden hose...
3) Then, you would dry the jeans (with you still in them) by riding your motorcycle to the bar for beers (and maybe a blowjob). California weather is ideal for this, but it was possible seasonably in other places. Note the position astride the saddle pushes the business out to help mold drying the fabric to the bulge, and helps adjust (stretch) the rear to any noteworthy anatomy there.
4) Repeat about a dozen times. Do not wash during this period, except for the wet then wear-dry sequence. You will end up with a pair of jeans custom-fitted to you anatomy (muscles and crotch) without any silly spandex look.
Some of my friends neatly took care of the wetting part by simply pissing the jeans while on the bike, but that's more of a hardcore biker thing... But it does allow a second wet-dry cycle, on the way home from the bar, and speeds your break-in time..