iCloud photo storage

The photos on your iPhone may be "Optimized". See the following from Set up and use iCloud Photos - Apple Support

iCloud Photos keeps all your photos and videos in their original, high-resolution version. To save storage space on your device, you can turn on Optimize Storage.

If you turn on Optimize Storage, iCloud Photos automatically manages the size of your library on your device. Your original photos and videos are stored in iCloud and space-saving versions are kept on your device. Your library is optimized only when you need space, starting with the photos and videos you access least. You can download the original photos and videos over Wi-Fi or cellular when you need them.

It applies to all iPhones, iPads, and Macs. Whether or not you turn on optimized photo storage is an option for the Photos app on each device.

Also, the amount of optimization will vary from one device to another depending, in part, on how much free space there is on that device.

MWINIJJ wrote:

It's possible that iCloud Photo Library is storing your photos in a more optimized format than the originals on your phone. When you enable iCloud Photo Library, your photos are uploaded to iCloud and may be converted to a more efficient format to save space while still maintaining high quality

iCloud always stores photos in their full size. Optimization takes place, if enabled, on devices but not on iCloud.

It's possible that iCloud Photo Library is storing your photos in a more optimized format than the originals on your phone. When you enable iCloud Photo Library, your photos are uploaded to iCloud and may be converted to a more efficient format to save space while still maintaining high quality

Does this apply to all iphones?