It ain't Paradox tho!
Cities XXL, of the Cities XL series, is a Focus developed and published game. (even tho, Cities XL was developed by Monte Cristo IIRC)
Cities: Skylines, a follow-up to the Cities in Motion series, is a Colossal Order developed and Paradox published game.
They don't share anything, except the misleading titles.
Even more, Colossal Order (and Paradox) is currently leading an extremely consumer-friendly approach, releasing big patches of amazing free content along with paid DLCs of almost aesthetic additions, basically making you pay for it only if you actually want to support them for their job, and not because you'd miss some important new content.