You just compared a measley 5 minutes to, actually more likely on average, 20min+ match of both GRB or ARB.
Comparing the two modes in such a way just does not work.
But yes, “activity” is too vague.
i dont know what game you play where matches are 20+ minutes. most ARB matches at top tier are over in 7 minutes, and GRB is more like 10-15 max. Sometimes yes you get the rare full 25 minutes match, but i havnt had that in like a 3-4 weeks atleast.