Which sounds good but just isn't true. At least for me. Maybe it is because the 9mm is a tapered case and the portion of the case behind the mouth is being sized to mouth dimensions? I dunno. But my 9mm and 30 carbine both have the wasp waist look to them.
But anyway, for me, the 38 plug does expand the case, in addition to flaring the mouth. I do t remember the exact dimensions anymore, but I pulled bullets before and after switching to the plug, and I was getting case swaging before that I am not getting after. Most of my leading problems went away after making no other changes than switching from the 9mm plug to the 38sw plug.
Not saying everybody needs to do it, but for the oversized bore of my witness match, with my cast reloads, it was a game changer for me.