Well for now it looks like they only plan on three variants an -800, -900, and -1000. With the -800 being the smallest of the three. As for why they started out with the -900, I believe it is because that variant has the most orders to date.
Probably a lot like the A-320.
Made the 320 first (like a 350-900), then the smaller A318/319 (350-800) and larger A321 (350-1000)
Orrrr something like that.