From whats been observed, if the game's average MMR is equal to your exact division standing, you will gain or lose 20 LP. When the average MMR of the match is below your division standing, you win gain less than 20 LP, but lose more than 20 LP. The opposite also applies when the average MMR is above your division standing, you will gain more than 20 LP, and lose less than 20 LP.

This does not apply when approaching promotional series, everything is reduced.

Also keep in mind that even when you are winning, the average MMR might not always go up. Based on your position in the matchmaking, its possible to win a game as last pick, then win another game as first pick, because the average mmr is lower and your league standing is higher, you might not gain as much