Muffler for my Generator??

[Jack & Donna Smith]

Ok..Enough..We are all Birders Togather.........
I need some basic info on mufflers. Our old 6.5 Onan gave up the
ghost, So we had a Emerald 5.5 installed.
(still have the 6.5 for electrical parts)
Problem is they connected the new Onan to the old muffler. It works,
but has a loud "Bark" to it. Not too bad inside the coach, but I am
afraid that I may camp next to someone bigger than me!!
Now I need a lesson in muffler engeneering. I know a straight thru
like a glasspac won't work, for obvious reasons. Will a larger
muffler be quieter than a smaller one, other things being equal? In
other words, Will one from a (say) 120 cubic inch engine be noiser
than one fron a 250" engine.
I am sure other factors involved, but I would like the Generator as
quiet as possible,,,,
Jack Smith
1973 FC 31
In HOT SoCal