The third button from the top in the control sidebar is the View Mode button . There are two options or modes for viewing images while in live view, Zoom to Fit and Normal View.
In Zoom to Fit mode, the image is zoomed in slightly so the height of the image fills the screen. In this mode, some of the sides of the image are hidden from view.
In Normal View mode you will see the entire image the camera is capable of capturing at the current location.