Let's imagine a simple, specific example: you want to add a button that sends a message to the shared backend and displays an alert upon success. Let's walk through the code.
In your web app, you could add an HTML button, style it with CSS, and then use jQuery to send a request and define your success handler. Probably not more than 10 lines and a few minutes (depending on much time you spend with the CSS).
In your iOS app, you could add a button with Interface Builder, link it to a method on the relevant view controller that sends an NSUrlRequest, then add a delegate method that displays a UIAlertView when the response returns successfully. Again, not more than 10 lines. And no CSS, thank god. :-)
Where is the extra overhead that you're talking about coming from?