TLDR: you have to turn off iCloud backup of your photos, and reinstate it, to get the free space and the size of your backup to update (i.e. reduce) when you delete photos from your phone.
Disclaimer – if you delete precious photos on my advice, I accept no responsibility! Backup all your photos first!
If you have an iPhone and you’re like me, and everyone else, you take a lot of pictures. A lot has been said, and probably a lot written too, about the finite capacity of the iPhone, the fact that 8Gb of flash isn’t a lot, when is Apple going to bump up to a minimum 16Gb, when will 128Gb be available and so on. It’s not clear if Apple’s rather measly and nonsensically expensive additional iCloud storage is an attempt to make money out of selling smaller capacity phones. As these undoubtedly sell the best (they are the cheapest), it may be.
Anyway, most people have the dilemma that you want to show people pictures on your phone so you don’t want to delete them. I copy all pictures to iPhoto on my Mac, but then they are much harder to show to people. Anyway, my backup got to about 4.3 Gb with 3.8 Gb of that being about 900-1000 photos on the phone (4Mb each, not hard to do the math). I started to get emails from iCloud warning me my storage was full, and that I should delete stuff or buy more. So, I checked as best I could that all photos were copied – this took a while and I couldn’t be sure. Then I started to delete photos from the phone.
I then went back to Usage, under Settings – General, but the backup was still the same size. Not quite trusting my own maths, and wondering if the photos were very small and somehow deleting them didn’t help, I deleted a bunch more photos. No change. I was getting a bit frustrated, wondered why this was happening. I seemed to be following the rough instructions…So I flicked the switch to turn off the photo backup altogether. Even though I knew most photos were on my computer, it still felt weird.
Flicking this switch back over to “green” was followed by a rapid recalculation of the backup size, and thankfully, a much reduced volume.
Bug or feature? I’m not sure. It might have reverted or recalculated over time. But in any case, no more annoying emails from iCloud. You mileage may vary, but even with only an 8Gb iPhone, I am still able to carry hundreds of photos around with me.