Another reason the size does matter (honest smaller is better!):

On the 1st gen Amigos, when we need a new exhaust system, we try to run it straight down one side of the floor pan. The original exhaust has the muffler on one side, the cat on the other and crosses over twice.

The shorter aftermarket muffler and cat, make this work, where as with either stock component, there just isn't room to make the fix.

That said, re-routing the exhaust probably isn't legal in [edit]many[edit/] states.