Most likely, your Mac display is of a different resolution than your iOS device. Screen Sharing, the VNC service that Apple provides with macOS, will not resize your Mac display when you connect to it from another Mac or iOS device.
You can use an app such as Display Menu that will let you resize your Mac display to match your iOS device display or at least select a resolution that closely matches it.
Additionally, you can also hide the Shortcuts toolbar, either via the Screens for iOS app settings (More Settings > Initial Toolbar Mode > Fullscreen) or toggle modes by performing a 2-finger double-tap once you are connected.