Just read an artical about it
One of the most significant improvements is in the video department, as you can now shoot videos in full HD at 60 frames per second (FPS), in addition to 30 FPS. Shooting videos at higher frame per second allows you to take much sharper and smoother videos.
But by default the iPhone 6 Plus and iPhone 6 records videos at 30fps. Apple has probably done this to ensure that users take an informed decision as the higher frames per second also means the videos will take up more storage space. Just to give you an idea, a 1-minute video at 30fps took up 130.4MB storage space, whereas a 1-minute video at 60fps took up 196.5 storage space (approximately 50% more storage space).
it will just take more space ^^ what is Common sense to be honest