Unless it is a game, apps don't automatically launch full screen for obvious reasons. It's a preference of the user to resize the window as they see fit for their own needs. If your app only supports iPhone then it will always launch using the size of an iPhone device. You can go back to the drawing board and include a made-for-iPad target and layouts or include native macOS support if you wish for your users to have the ability to resize the app window.
Topic:
UI Frameworks
SubTopic:
SwiftUI
Tags: