You can delete videos from iPhone to save space.

Delete a video:
In the videos list, swipe left or right over the video, then tap Delete.

When you delete a video (other than rented movies) from iPhone, it isn’t deleted from your iTunes library and you can sync the video back to iPhone later. If you don’t want to sync the video back to iPhone, set iTunes to not sync the video.

IMPORTANT: If you delete a rented movie from iPhone, it is deleted permanently and cannot be transferred back to your computer.