Lots of glass packs have a diameter reduction after the pipe gets into the body of the muffler. Many also have directional divits or holes stamped inwards that can either mildly hinder or moderately hinder flow as well depending on which way you face the divits inside the tube.
Many race type mufflers are glass packs which do not narrow down while the tube runs thru the body of the muffler and the holes in the tube inside the muffler are drilled holes rather than stamped flutes or divits that protrude into the tube.
A glass pack muffler which is designed and sized properly is actually a tuned chamber which can reduce drone considerably, secondly the packing or wading absorbs other frequencies and volume in the exhaust note.
That being said, glasspacks sound great when used properly. I had a pair of 32" glass packs duals which were 2.5" at the center of the tube id inside the muffler and 3" inlet/outlet. They were pretty darn quiet.
Part of what makes a good sounding exhaust is the piping. A smooth route, good welds and good smooth flow, running out to the bumper. On the other hand lots of irregularities, weld protrusions and globs inside the pipe, transitions in size and harsh bends can add a fluffy wheeze to the sound rather than clean clear note.
Because most normal glass packs neck down at least 1/2 " or more when the tube runs inside the muffler, this causes a reflection wave back up the pipe towards the engine which can f-up velocity, scavenging and flow. Maybe that is why they sound like they do, which is to say, they sound pretty sweet.
. But interior resonance is no fun at all - will change to a set of magnaflows this winter. 
