Not sure if 3” is the norm, but the DA does so many things so well that I don’t think many people buffing cars need anything bigger. DAs are so good they seem to be the industry standard as well(at least for hobbyists and pros, not so much body shops). What a rotary does better than a DA is, tight, confined edge work. My opinion of course.
Having no “throw” like a DA, 8mm, 15mm etc means you can place it right up against trim, under mirrors, polish mirror caps or to polish hood vents for example. Not to stay a DA couldn’t handle that type of job, but the rotary with smaller pads becomes safer to use as well, when pads become smaller on a DA, it becomes more aggressive.
One last point, a smaller overall frame will accompany a polisher built to support 3” pads as opposed to a PC style, GG6/G9 DA made to turn 5”/6” pads. This makes it easier for the user to control since it’ll be lighter weight, have a more compact design and fit those tighter places better.
