What makes a "chuuni" anime?

Most of us already know what Chuunibyou is, and what it means when applied to a character.

Now, a few months ago I came across the term "Chuuni Anime" to describe a specific type of anime.

The term didn't seem to refer to anime that star characters with Chuunibyou syndrome, like Chuunibyou demo Koi ga Shitai or Inou-Battle wa Nichijou-kei no Naka de.

Rather, "Chuuni Anime" is now commonly used to describe stories with settings that are impossibly big, overcomplicated and full of powerlevel. Or as someone else put it, a anime with a setting that people with chuunibyou would like.

Examples seem to be anime like the whole Fate/Franchise, To Aru Majutsu No Index, or Shakugan no Shana.

Now, here's the question: What's the difference between a "Chuuni Anime" and a normal sci-fi or fantasy anime? Or with a typical shounen anime?

Most people seem to agree that shounen anime like One Piece aren't really chuuni, despite sharing all the traits that make a chuuni anime.

Furthermore, there's anime like Durarara! or Black Lagoon that aren't described as chuuni at all, despite also sharing a lot of traits, such as superpowers, powerlevels, or complicated settings with a focus on action. In fact, I read a thread somewhere else where people were arguing about whether Psycho-Pass was a chuuni anime or not, and that only served to confuse me even further.

So I'm curious about what you guys think. Have you heard of the term chuunibyou to describe anime? If so, what do you think is the line that divides a chuuni anime with normal fantasy/sci-fi anime?

What makes a "chuuni" anime?