2.5" exhaust vs 2.25 or even 2"

My '69 340 Dart came stock with 2 1/4" duals to the mufflers and 1 7/8" from the mufflers back.I thought the 383 GTS would be the same but i've been wrong before?
And dibbons is right...2 1/2" are quite a bit larger and heavy too.