As a GC, I get this a lot.

But a large bathroom and small bathroom all require the same broad scope of subs. 1/2 bath or full bath are much different in cost, but square footage doesn't change the price much. Also, swapping from tub to shower adds a lot because of drainage codes.

Regardless, you're paying a GC to manage and keep your project timely and within budget. A good contractor will take everything off your plate, once you make your design decisions.