Sometimes when you add video from multiple sources to a project, all the video doesn’t fit within the aspect ratio that you’ve assigned to the project. To address this issue, you can set iMovie to automatically crop added video to fit the aspect ratio, or to fit added video within the frame, even if it causes letterboxing (black bands on the top and bottom of the video frame) or pillarboxing (black bands on the sides of the video frame).
To automate cropping:
Select a project in the Project Library, or open a project, and then choose File > Project Properties.
The File menu appears in a light gray bar across the top of your computer screen.
Choose one of the following from the Initial Video Placement pop-up menu at the bottom of the pane:
Crop: Fills the frame with as much video as possible, though the entire video image may not fit within the aspect ratio you’ve set for the project.
Fit in Frame: Fits the whole video into the project’s aspect ratio, even if it causes letterboxing or pillarboxing.