It's a proxy for complexity.

Not the op and I get your point, however...

One way to think about it might be that the site supports lots of users who use it for various things. So, everyone uses 80% of the site, but everyone also uses a different portion of the final 20%. So, if you have lots of users, you might also have lots of smaller features that a significant minority use.

I don't know, just an interesting way of thinking about it.