If you have a Windows machine lying around, the iCloud app lets you download all your media out of iCloud Photos. Once it's all on Windows it's a lot easier to find the largest files!
You can also do that with Photos (leaving "optimize storage" off) and downloading all to your Mac, I believe. I haven't actually done yet that since it is quite a large download.
the windows icloud app doesn't download the highest quality versions of each file though right?