It looks like you recorded your videos while holding your iPhone in the vertical position. When you do that the video will import into iMovie in a vertical 9:16 aspect ratio. iMovie's screen has a horizontal 16:9 aspect ratio and therefore black bars are inserted on each side of the 9:16 video to make it fit the 16:9 screen. There is no way to change the iMovie screen dimension.
However, you can easily fix the problem with a workaround. You rotate the clip sideways. Share it out. Open it in QuickTimePlayer and rotate it right side up again. Then save. No more black bars.
In more detail: Put the vertical clip into the iMovie project timeline. Click on it to select it. Then click on the crop icon (the one that looks like intersecting angles) in the tool bar at the top and to the right of your screen. In the controls that appear, click on the rotate control (the little tilted box with the rotate arrow next to it). Click on the rotate box to rotate the clip sideways. Then click on the blue reset button to the right of the screen to apply the rotatation. Then share out the clip to your desktop. Open it in QuickTimePlayer. Do Edit/Rotate to rotate the clip right side up. Then save to desktop. That will eliminate the black bars.
-- Rich