iPhone 6 memory usage on photos

Hello,

I have an iPhone 6 64GB.

When I go to SettingsGeneral/Usage/Manage Storage it shows that 27.8 GB is being used for Photos & camera. This is a huge amount considering that when I access my phone via iTunes it shows only 3.72 GB is used on photos with a total of 12,171 photos. iTunes does show 24.78 GB "Other" storage.

Is this discrepancy something to do with the way iCloud Photos shares photos with other devices? My iPad shows a similar discrepancy i.e. 21.2 GB on the iPad but only 1.07 GB via iTunes with 22.10 "Other".

I would be really grateful if somebody could explain. If iCloud Photos is the reason, I may have to look elsewhere for a Photos solution.

Many thanks