Okay heres another example. Shadowlands season 1 was flooded with players because pvers were gearing up through pvp. The participation was massive, but the rating was deflated.
Blizzard then tunned the knobs to avoid the same issue in Shadowlands season 2, but overshot the mark and it was massively inflated. Thats when i got duelist in 2v2 with a friend despite never queuing arena.
S1 had massive participation and was deflated, S2 had less participation (pvp gear ilvl was nerfed in pve) and was massively inflated.
Blizz said they were going to make changes to mmr in DF season 2
Rating distribution is not wholey dependent on participation, blizzard has knobs they can tune to adjust the bracket.