Photos will copy everything in the library even if told not to?

[Romf]

So, I have an external hard drive with all my pictures.
I want to use Photos to manage all this, keeping the Library file on this external drive, and without copying all the photos in the library (so I don't use twice as much space).
If I look at this link:
http://www.imore.com/how-use-external-and-referenced-libraries-photos-os-x
It should be posible, and the option in preferences is pretty clear about this:

Photos technically supports management of images that aren't locally stored within its library — that is, you can keep a set of images in a folder called "October Trip" and manage them within Photos without having to make a separate copy of them. You can do this by navigating to Photos > Preferences from the menu bar, selecting the General tab, and unchecking the "Copy items to the Photos library" box under Importing.

BUT, when I start a new library after unchecking this "Copy to photos library" and add a few folders to Photos, the library grows to approximately the size of all these folders combined...
How to avoid that? Is this a normal behaviour or a bug?
Thanks!

 

[Romf]

And, attached, a screenshot of Photos help that says it should be posible