ORIGINAL: bjr_93tz
What they mean by that is today's 40's can probably spin a bigger prop at lower rpm than what the old 60's could which is great if you want to fly around at 30-40mph, hang on the prop and do 3d stuff, but the 1000ft loop at 70-80mph still needs cubic inches (you need at least a 500ft loop to do a proper cuban 8)....
I think there's more to it than that. I've heard that the physical size of engines changed at some point. The old 60 became the new 40. The new 60 became a larger size altogether.