The unused part of a clip are called the "handles". If you have handles on a clip, you have two choices when making a transition. You can keep the length of the movie the same, and use the handles for the transition, or you can shorten the movie and make the transitions out of the footage you have chosen.
The parameter is called Transition Overlap. Set it to Half to maintain project duration. Sut it to All to maintain the clip range, but shorten your project.
See this section of imovie Help for details. Click on "To set how transitions overlap with adjacent clips."