Exactly what i was thinking! I mow a regular lawn that takes 1.5 hours, fortnightly mow in summer, kikuyu, and I might get 6 chaff bags. Honda 3rd gear, edges all double cut, same method as you Happy with two blows etc.
4 cubic metres of grass caught, walked and emptied into trailer etc etc is a friggin shedload of grass believe me. Do you have pics?