Assuming they are both listed as the same size, inlet will actually be larger.

 

I'd think the inlet should fit inside the pipe coming from the manifold. Anyway, see the diagram I updated with. That should determine it.

 

The inlet pipe should go inside the fitting of what it is plugging in to. At least, that's the way I was taught to do it. Theoretically, it's less likely to leak that way. (of course, we welded the pipes all the way around, so leaks simply weren't an issue in any event....)

 

We agree on that. This is why I was guessing it would be smaller. Im talking the inlet size on the muffler itself, not what it connects to. In the same vein, the outlet should have the exhaust pipe inserted into it, so it would need to be larger.