We've been looking exclusively at Xamarin forms where the user interface is designed in the cross-platform Xamarin project. When Xamarin was initially released, and for some time, the app's graphical or presentation layer had to be created in the platform-specific project. Xamarin was only used for the app's logic and data layers. Because Xamarin forms are confined to APIs common to both platforms, like buttons, dropdowns, and other standard graphical elements, you are necessarily restricted in how the user interface can look. In one respect, this is good because you probably want the user experience to be the same everywhere. On the other hand, if a platform has a control or some hardware-optimized feature and you want to take advantage of that, then a native UI is the way to go. This is particularly true of games with heavy graphics processing or apps that make extensive use of the device's hardware like the camera, GPS, or microphone, where there is constant use of native APIs. 

Depending on whether we're talking Android or iOS, Xamarin forms apps are two to three times larger than Xamarin native apps when deployed. Both types of Xamarin apps are at least an order of magnitude larger than purely native apps. Performance wise, Xamarin forms apps will be slower to load than Xamarin native, which in turn is slower than pure native. The difference between pure native and Xamarin is significantly more evident for Android than iOS, which you can attribute to the MonoVM used for Android apps – remember, Xamarin produces ARM instructions for iOS. Having said that, most users won't notice a performance difference between a purely native app and a Xamarin native one. When it comes to Xamarin forms apps, you need to consider the app's complexity, especially the UI complexity, versus development time and the need to have a single codebase.