How you win— by checkmate or by a resignation— doesn't matter.
What matters is 1) whether you win, lose or draw and the difference between your rating and your opponent and 2) a statistical measure that looks at whether your rating is likely to be correct.
You get 8 points or lose 8 points in a game where you're playing against someone of roughly the same rating. If your opponent is higher rated than you, I think you can get up to 12 points for a win and would only lose 4 if you lost (not 100% sure but I think thats the max).
The games where your opponents are getting 20 points is probably because they don't have a solid rating yet. You'll lose and gain a lot of points per game where you're new to the site and it's trying to place you in your correct position
i only ever gain and lose 8 elo for each game. Even when i checkmate in a few moves or play horribly. everyone who dont play as good at me get in the 20s. any reason why?