I think the word "proper" as it's being used in this thread means more like "appropriate" or "acceptable" -- e.g., "what American restaurants consider a proper serving." Not that other portion sizes are "improper" -- just different.
Some restaurant meals are huge, but they aren't standard everyday fare. That humongous Denny's breakfast may be the only real meal a long-haul trucker eats all day. For many of us who aren't truckers, the big restaurant breakfast is an occasional treat, usually on a weekend, when (a) there's time to eat it, and (b) we're getting to it later than we would breakfast on a weekday -- because we've slept late or gone to church beforehand. Essentially it functions as brunch (combination breakfast and lunch), because nobody's going to be hungry at noon after eating a ginormous breakfast at 9 or 10 a.m.
Which has me wondering: How common is "brunch" in other countries? Here in the U.S. there are places that serve Sunday brunch buffets that include both breakfast and non-breakfast foods -- eggs and bacon and bagels, etc., but also a roast of some kind, pasta dishes, seafood Newburg, veggies and such. The classier ones also serve alcohol -- usually bloody Marys and mimosas. They generally serve from 10 a.m. to 2 p.m., so you can get breakfast food, lunch food or both, depending on what you're in the mood for.