I believes it calculates the score change for a person, by comparing the person in question's performance against all other players in that game. If a weaker player holds their own against a stronger player, the weaker player does well.
Imagine three players:
Player A (1000)
Player B (1500)
Player C (2000)
They play a 3 player game (WTA): Player B eventually comes out on top.
The score is then calculates score change for each player (the following is an estimate)
-Player B
i) BvA, B gains a few points for beating player A, but not many say 4
ii) BvC, B gains several points for beating player C, say 15
B would gain 19 points.
It follows that if you play a game with more players you have more elements to sum, and therefor a greater score gain.
ex: https://www.vdiplomacy.com/hof.php?
gameID=44844EX