Why are vents sized the way they are

Vents provide a method of expelling sewer gasses out of the house, which can be explosive and smelly. They also let air displace water as its flushed down the drain. Now there is no way a typical home has enough air displacement to justify a 3 inch vent. They are 3 inch or whatever size because that allows a plumber to get a plumbers snake into the main drainage. A lot of modern homes have only 2 inch vents throughout, simply because its less expensive to do so. If you look at a lead flashing over a 1 1/2 inch vent, you are lucky if you have a 3/4 inch hole. Our vent shields are designed to provide sufficient air access for vent function.