Ion capture amounts increase with the radius of the funnel, literally fueling higher IonJet burn rates. So while a 1km radius near Earth produces 16.1 pounds of thrust, 2km produces 64.4, and 10km produces 1,611 pounds! Ship classes are defined by order of magnitude groupings based on the average efficiency out to 500km. Yes, 500 is completely arbitrary and I’m about to commit a well known mistake, but at this point I can’t imagine anyone building larger than that for the next 200 years or so, so it’s good enough.