Turn off clip stabilization. Stabilizing clips causes a zoomed in effect, such as you are experiencing.
Double-click on the clip tp open the Inspector. In the Clip tab, uncheck the box labelled "Stabilization - Smooth clip motion".
Also, in the menu item iMovie>Preferences, in the Browser tab, uncheck the box labelled "Automatically stabilize clips that have been analyzed".
When importing clips to an Event there is an option to analyze them for stabilization. It's best to deselect this option, as this will speed up the import process. You can always analyze selected clips after import, then stabilize them using the Clip Inspector item once added to your project.