What makes the rpi gtm sound so loud?

[Craze]

So i am catless, and looking under the car there is only resonators which dont take away much and then straight pipe to mufflers which when valves are open are straight through

So what makes the rpi gtm exhaust so loud? Is the pipe larger dia and the x pipe less crushed than factory??

Debating if i should get gtm exhaust or custom made? Nut gtm exhaust to get to australia is very dear as no dealer here!

Thanks