...you do understand that in that case, the number of played games overall doesn't change, right? They are still playing 75 games, just spread between 5 players instead of 15

And yes, a smaller clan is inherently going to have to have less active players than a larger clan. In the grand scheme of things, and I'm unsure about how you are not understanding this, what matters is how many people play the game, not how many games are played (grand scheme of things being toribash as a game trucking along). Player count is more important than games played, just kinda, universally in games, y'know.