I don’t know when this change happened but it used to be that if you picked “presentation - fill” the prototype would fill by WIDTH of the design. Now, very confusingly, it fills by HEIGHT. So if you are designing a website, and you pick fill, it will fit the entire page in the view finder. There is a “quick fix” in the viewer where you can select “fit width”, as I’m sure there were a lot of complaints about this, but now, when sending a prototype to someone, you have to add screenshots and instructions on changing Figma presents along with it.
Please add " fit width" to the prototype section in the designer BEFORE you are in viewing mode so that the prototype looks correct on load!