I think there is a sticky on one of the forums, about bits, isn't there? tells all about them. might be worth looking for that.
you have to spend some time envisioning how the bit fits in the horse's mouth, and how it moves when a pull is put ont he reins and such. imagining that will help you see why a snaffle with a twist would hurt more. Since th bit pushes down on the hrose's bars (basically , the gums of his jaw), anything with a sharp edge will hurt more, and a thinner bit will apply the same pressure over a smaller area, thus more psi and more pain. physics, just like you said.
Some horse really hate the single jointed snaffle because it pinches the tongue, while others hate the double jointed becaause it applies more downward pressure on the bars and top of the tongue.
But ultimately, its the hands of the rider that will make even a gently bit harsh, and a severe bit delicate.