The downpipe is 3" so thats what fits...
That or its easier to just put the cat on and leave it there rather than worry about a $1000 fine and having to put one back on every two years for an e-test.
In Ontario, there are people who drive around and inspect cars they suspect of having missing pieces like a cat or EGR valve. They do hand out tickets that are valid as the law does state removing emissions equipment is illegal.