PHOTOS are typically the biggest consumers of storage space in iOS devices but not necessarily in that order. However if you are not allergic or otherwise opposed to iCloud Drive, Apple has help for you in the form of optimized images stored on your iOS device. An optimized image has been adjusted in size, color depth, image resolution, etc. for optimum display on the iOS device screen and if taken using iOS 11 & 12 may be in Apple's revolutionary HEIF format (smaller file higher resolution). The original full size image is stored on iCloud and when you edit, print, mail, that image from the iOS device the full size image on iCloud is used, not the optimized image. That is all totally transparent the the user.
A good idea although I am somewhat "allergic" to iCloud. As well, this would keep me from doing what I should do....Manage the content. i.e. turf a bunch of stuff that shouldn't be there.
