Apple isn't very transparent with how they deal with iCloud Photo Library and Optimize device storage.
They want to keep photos stored on the device for 2 reasons.
First, its a better user experience to have the photos stored locally. This way when you want to see them, share them, edit them etc etc you aren't reliant on your ISP or cellular providers. I used to have a 16gb device and it was a pita with a large library since it would always be downloading the original from the cloud and if network conditions were bad enough sometimes it just wouldn't work. Now with a 64gb device even though only a small percentage of my total photo library size is stored on the device I feel nearly all the photos I look at "feel" like they are locally stored.
Second is to minimize cellular data usage. Since Oct 28th (last reset) Photos has used 475mb between syncing and me downloading original versions of pics. This would only get exponentially worse if they moved more photos off my phone. Since I have plenty of space on my phone that would just be completely unnecessary and wasteful for my data plan.
Sometimes (but not always) manually syncing with iTunes will reallocate the cached photo data to the "other" section of the phones storage. However this is probably just a visual change and that data was already set as cached and scheduled for removal if storage space was required.
I just sync'd my iPhone to iTunes the first time in eons and my photo storage went from 5.6gb to 761mb. However again this space wasn't reclaimed, it was just reallocated to the "other" section in the phone. I mentioned this doesn't always work because I was attempting to follow the data on my iPad however syncing made no difference. The variable there is I always look at older photos on my iPhone and I never look at older photos on my iPad so maybe nothing was cached on the iPad.
Regardless you have more than enough space even at 64gb. Its a moot point anyway since with iCloud Photo Library on when you sign into the new device the photo library will repopulate with the majority of data not downloading to the extent it is now.