react native doesn't fill up window on ipad

Solution: In the xcode project, go to the general tab and then to the Deployment Info. There you can change to the device type you want (universal works for both).

If you are using Expo (create-react-native-app), modify to your app.json to set ios.supportsTablet to true:

"ios": {
    "bundleIdentifier": "your bundle identifier",
    "supportsTablet": true
},

If you are using react-native-cli instead of expo, you can open your .xcodeproject with Xcode. Then go to Build Settings Tab -> Targeted Device Family -> Select Universal.

Next time you run your project with "react-native run-ios" targeting an iPad device, it will display the app in fullscreen.