The beak on a HR piece can be both broader and have a "taller" bite -- which I tend to like as it seems to help me play with a rounder, more "open", less "biting" embouchure. Of course there's probably a reason for the different beak design, influencing the way your embouchure is set up to go for a different effect (sound). This in addition to the (probably) different internal dimensions. Also of course, it's possible to have the same internal dimensions between the two (I believe the Ponzol HR Custom is an example that is similar to one or the other of the Ponzol "M-series" metal pieces), and yet one is made of HR and the other metal. Chances are the HR will still have larger external dimensions.