You might be experiencing a zoom if you have the Ken Burns effect turned on. Select the image in the timeline and then click on the crop tool (the one that looks like intersecting angles) in the tool bar at the upper right of your screen. A style selection box will appear to the left. Click on "Fit". That should remove the zoom. For future projects you can change the style preference in iMovie/Preferences/Photo placement. In the box you can select "Fit". In projects started thereafter, "Fit" will be the default.
Not likely but another possibility is that your photo may have some odd dimensions (aspect ratio) that iMovie is expanding to fit the 16:9 aspect ratio of iMovie's screen.
-- Rich