If you turn on iCloud Photos and the Optimize iPhone storage option on your phone, photos on your phone may occupy less space than on iCloud. See the following from Set up and use iCloud Photos - Apple Support
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. You can use Optimize Storage on your iPhone, iPad, iPod touch, and Mac.