Drew Coates said
Wet debris, MPH is more important. Dry debris, CFM is more important.
Both your backpack and your billy goat are 200 mph but the 18 hp walk behind is probably 5 or 6 times the Cubic Feet per Minute. So it will move a lot more debris/bigger piles, which you are seeing.
The Hurricane and Brown claim around 6-7000 CFM I guess. That may be twice as much CFM as the Billy Goat walk behind. Hard to say because they seem to be avoiding a specific number of CFM with these high volume machines.