Same issue. Glad to see I am not the only one.
iPhone 11 Pro Max to iPhone 13 Pro Max using phone to phone data transfer on Friday. Photos took up ~66GB on the iPhone 13 Pro Max and making the phone storage 126GB used out of 128GB capacity. I thought it might just take a few days to index/optimize, but after 3 days, it's only gone down to ~60GB, and my phone is now indicating 123GB used out of 128GB capacity. I thought with my storage being this low, the phone would have been a lot more aggressive in optimizing the photo library.
For comparison, my iCloud photo library is at 75GB. The photo library on my iPhone 11 Pro Max was at 6GB (and the phone was at 84GB used out of 256GB). The photo library on my iPad Pro is at ~10GB, with the iPad storage at 133GB used out of 256GB.
So somehow on the iPhone 13 Pro Max, despite my phone storage so close to running out, the photo library is still not optimizing properly. I have left it plugged in for most of the time the last few days, and plugged in overnight while I sleep. Hopefully it will sort itself out in a few days.