Turn off Stabilization. When clips are stabilized in iMovie, a slight zoomed in effect is caused.
Double-click on a clip to open the Inspector. In the General tab uncheck the box labelled Stabilization: Smooth clip motion (if checked). See this screenshot:
It's also best to uncheck a preference in iMovie's Preferences. From iMovie's menu select iMovie > Preferences. In the Browser tab, uncheck the box labelled "Automatically stabilize clips that have been analyzed" (if checked). Also, when importing clips to an Event don't select the option to analyze for stabilization - you can always choose to analyze after the clips are imported. For the preference item, see the following screenshot: