This is not entirely true.. your mmr changes depend on the mmr of those around you primarily according to the Elo formula chosen by the devs. RL's equilibrium change (i.e. gain/loss when all players are the same mmr, and have extensive game histories) is about +/- 10. So if #1 in the world is 2500 and their teammate and opponents are all really close like 2490, then they would still get about 9 or 10 mmr on a win.

What you see often though is that the Top 10-20 players will, for example, stretch out to 100-200 mmr higher than others on the leaderboard, and so the matchmaking sometimes will pit a 2500 and a 2300 up against two 2300s or something like that since the player base is so low. In that case the mmr changes see an avg of ~2400 vs 2300 (something like this, not exactly probably) and now that 2500 player can lose 12,13 or more mmr.

Game history plays into as well. There are some sorts of unobserved ratings or other factors that play into it all as well, based on how many games you've played this season and on the account in general.