In the über-cush world we find ourselves in, we may sometimes like to imagine that a heavy backpack can be worn over many hours and many trail miles with no discomfort. As some of the techies and engineers who design this stuff (not to mention that we test our packs out in the wild, often with quite heavy loads, and like comfort as much as the next guy) we often hold that up as our Holy Grail: a pack that carries as comfortably as nothing at all. In the not-too-distant past the world had porters (some places still do) and the future may offer anti-gravitation technology (other planets may have this already), but for now we just try to make the shoulder straps on our backpacks as comfortable as possible.
We've been using die-cut, thermo-formed shoulder straps made of closed-cell EVA foam for a very long time. We've refined and further refined the shape of the curve, the width of the pad and thickness and type of foam. Over decades of theorizing, field testing and actual production, we've come to the conclusion that the shape of a shoulder strap is far more important than the thickness of the foam padding. In fact, in our humble opinion, a heavily padded strap can sometimes mask what is essentially an ill-fitting strap, the thickness of the padding essentially making up for a lack of nuance in the shape.
Here's some of the hard-won trail wisdom from us life-long pack builders and users:
Adjust your straps to fit you, but also adjust them during the course of the day. You'll want to distribute the weight of the pack over different muscles/soft tissue over time. Do this, of course, by small adjustments to the length of the lower strap. But use the sternum strap to fine tune and modify the fit over the course of the day as well: tightening the sternum strap will bring the shoulder straps closer together; loosen or unclip the sternum strap for a while so the shoulder straps can ride a bit further apart. Use the waist strap for a while, then loosen or unclip it for a different ride. If you watch closely, most experienced hikers are constantly performing these micro-adjustments as they walk -- after a while the adjustments become as unconscious as shifting weight to one foot or another when standing for long periods or alternating reclining with sitting up straight while sitting for long periods.