The answer is they are exactly the same thickness. If you took any model bat on the market and stripped it of its end cap and all counter weighting in the hands, the "shells" would be identical. Bat manufacturers already spend enough on R&D and certifications and are not producing 100 different shells for their product lines. What they do invest in is their creative team to come up with 20 different designs to paint or wrap onto a shell.
Take for example Miken and Worth slow pitch bats. Both lines of bats are exactly the same, less the cosmetics. Miken and Worth slow pitch bats utilize 4 shells. 2 ASA and 2 USSSA, 1 being 12 inch and the other 14 inch. Every single Miken or Worth slow pitch bat you see, no matter the weight, uses a derivative of these 4 shells. This includes limited production bats and exclusives.
Well how can this be true if bats vary in weight? The variation in weight comes from end cap and handle weighting. You may see some videos online of people counter balancing bats on 2 scales to see where the balance point is. What they are essentially doing is seeing how much handle vs end weight is in the bat. It has nothing to do with wall thickness. Basically, the weight of each bat is offset by the end cap and handle weighting to achieve the finished total weight and balancing.
This same principle applies across the board for all makes and models. So the next time you hear someone talk about how heavier bats are thicker and more durable, you can drop some knowledge on them by debunking the ancient theory that heavier bats are thicker than lighter bats.