Not necessarily. The inside diameter has less to do with it then the actual volume of the bowl. For example, I have 2 Dunhills that are a group 2. The inside diameter of these pipes is about the same as a number of pipes that I have with much deeper bowls. I have another pipe with a somewhat smaller diameter then these two dunhills, but it is much deeper, and taking a visual measurement, I would say the volume of this pipe is greater then that of the Dunhills, so I would classify it as a group 3, using volume as my gauge.
As noted above, Dunhill has never let the outside world know precisely what their measurements for a particular group might be, so we just kind of take a visual of the pipe and we say "that's a group 3 that's a group 5" and so forth. It's not very scientific, but it seems to work.