At the start of Halo 3 it wasnt as widespread because of the population counter, it was so high that the MM in Ranked could have that flexibility of doing more suitable matches, once, in the case of MLG playlist, I got more reduced (bellow 14k) you would get paired with teams just as easy, like you said the game did had ways to show you it was against a team. In Halo Reach MLG the MM against team got bad, just as "bad" as Halo 5 is.
Evidence is maybe imposible to use based on game history, but, like i said I got to MLG 43 (i could be wrong it could be lower) playing solo, so I remember very well how I saw that one Bar in the opposing team, also, the higher the level the easier it was to find a team, remember that Halo 3 had the option to party up as well, so a lot of randoms could go into Halo and actually make a team in the post game report, I think Halo is missing such option.
Also, you got it wrong, the larger the player base the less likely you get to play against a team.