I actually have an old Rivarossi catalogue on my shelf, which cites something like a 420mm minimum radius for the Big Boy. I can't imagine it looking very good going through that, or being all that stable.
They needed to build it like that for the Euro market, because many of even the largest of our display and club layouts don't go much broader than 18-22 inches. Most of our locos and rolling stock is short compared to yours and consequently have less problems with tight turns. Architecture plays a part, too, with European houses being smaller and having a lot less basements or double garages. And, of course, the German hobby is extremely collecting-oriented, with less emphasis on operations and layout design.