It’s possible that this trick stopped working last year.In iTunes Store try to rent a movie which is large in size than the space left in your iPhone then you will receive a message that you do not have enough space. After this message you go to check storage space you will be surprised. I am using iPhone 6s Plus.
With a movie that large on a 16GB iPhone you'll have to delete everything on the iPhone to free up enough space for the movie. That means no music, apps, photos can be on the iPhone. Back up your iPhone to iTunes on your computer then erase it and set it up as a new phone. That should give you enough room for the movie.As I remember this trick used to work 3 or 4 months before. I have 2 GB left out of 16GB. Movie is 10gb large.
The trick is/was not to get enough room for the movie, but to free up a bit of storage space on the iPhone:With a movie that large on a 16GB iPhone you'll have to delete everything on the iPhone to free up enough space for the movie. That means no music, apps, photos can be on the iPhone. Back up your iPhone to iTunes on your computer then erase it and set it up as a new phone. That should give you enough room for the movie.