By default, React Native only targets iPhone - so if you run on an iPad, it will show up as a scaled app, and won't look like a native iPad app.
To target both iPhone and iPad, you have to change the xcode project to build it as a universal app. Do that by finding the Deployment Info section of the General tab in the app target - and switch that from iPhone to Universal.
Then, you can run your app in either the iPhone or iPad simulator, and change the styling based on the current device.
https://egghead.io/lessons/react-native-target-both-iphone-and-ipad-with-react-native