How about some sacrilege? The same amount of peanut butter cup but in a more efficient square shape. This is to save about 15% in packaging and increase the number of bars that can be shipped around the world.
But wait there is more: I ditched the black wrappers so the cardboard tray has a flap to keep the cups separated. This helps reduce more waste, plus those darn wrappers always tear some chocolate off the bottom of the cup!
This is another post about helping the environment, sorry! Besides the shape and removal of the black wrappers, would anything be different? Same amount of candy bar, same taste.
Here is a size comparison to wrap things up:
And yeah, other foods like OREOS could use the same approach. In my opinion OREOS should be rectangular like a biscuit, which would make for optimal dipping in milk.
Dave's Geeky Ideas 
I might be with you on the Reese’s, but no way on the Oreos…you’d lose that glorious ritual of twisting to separate them and lick the filling inside! Sure it’d kind of work with the new shape, but it’s much clunkier. I’m convinced there’d be riots if they went square 😆