The tricky thing
is that at the same time as lagom means
"Not too much, not too little, but just right
" it also actually means "Not much, not little, but just right (which happens to (always) be somewhere in the middle
)" i.e. baked into the concept is the conception that "the middle is always the best."
This is an ancient and hardly uniquely swedish idea: Aristotle believed in the "golden middle way" and Taoism is based on finding just the right measure between extremes.
The real bravade with "lagom" is more that the Swedes' minds accept this philosophy so completely that they use "this is lagom" when they in the moment of speaking haven't given a thought to if "this" can be characterised as "in the middle,"; they just want to say "just right." (Indeed, right now I can't remember any common Swedish expression that just means "just right".)
And conversely, especially in childish talking, "lagom" is used instead of "mittemellan" ("in the middle").
So when someone tells a Swede that "the more, the better," he may say "That can't be (logically) right, because Lagom är bäst (Lagom is best (Swedish proverb)); that's a tautology."
This can get you into locked-into-the-language newspeakish situations sometimes, like when you were a kid in school and the school restaurant cook who served you the food would ask you: "Do you want much, little, or lagom?"