In the US, code dictates that the drain (p-trap and arm) on a shower use 2" pipe, while on a tub you use 1.5" pipe.
I understand that is code, but I am curious as to why it is the way it is. Sometimes code just is what it is, but I was wondering if there is a physics answer to this question. Is there a reason why a shower is larger from a performance perspective?
And, is there any harm in using a 2" drain pipe for the tub as well?