If all you have room for in your bathroom design is a single sink, then do not despair. Even if you have to share the bathroom with another person, a well designed and placed single sink can still accommodate you both. What they did here was make the mirror big enough so that you can feasibly share it with your partner. I would hope that at the same time they also have a big enough shower stall for you to share if need be. When it comes to having a bathroom that you must share, but is a smaller space, it is all about building it with efficiency.