And, when looking at data from our testing, the larger and smaller configuration flows about 70% of a straight through 2.5" piece of pipe but the larger one is about 5 dBA quieter at ten feet at 3500 rpm and 2 to 3 dBA at 750 rpm. All were on a 455 cubic inch Pontiac engine. The larger volume really works and all muffler guys at the OEMs used this to tone down the big powerful engines when they wanted to avoid doing so by restricting the exhaust gas flow.
Really quiet HiPo setups used multiple mufflers with large volumes. The more the better if configured right.
This is why Chrysler used to suggest using the big 1971 or so Imperial mufflers. They worked well and it is too bad you can't find them any more. Before the advent of today's Hi PO mufflers (around the early to late 90s anyway) the old single exhaust Imperial mufflers were a super choice if you could make them fit.