The latest update for the Nike+ Run Club now fills in the larger 40mm and 44mm display on the new Apple Watch Series 4 models. Before this update came, the app just ran a smaller version of the app that worked with earlier Apple Watches. This time though, you get to take advantage of things like special Nike watch faces with fixed complications for launching the app. This optimization makes information a bit more glanceable when you run. Make sure you update your Nike+ Run Club app for the iPhone to get access to the updated watchOS version. This new update also brings a new tab to the iPhone for discovering new shoes and sportswear for running. 

Source: 9to5Mac