Have a look at the exhaust pipes on the California: Tiny isn't the word! Rich is right though, exhaust gas flow is more about design than actual pipe bore size. Given also that it's not a full-scale production car and doesn't have to meet all the silly noise/emission requirements, I wonder if there's a cat on there at all...
Shame it's so bloody ugly. We forgave the original as it suited the time period and had racing pedigree, but that monstrosity needs burning sharpish.