"Your full-resolution photos and videos stored in iCloud may not fit on this Mac." even with 45 GB of free space on internal SSD

Somehow I accidentally activated iCloud Photo Library and about 5 GB of my photos now reside there. Since I use other cloud services for my photos, I wish to take these off iCloud. macOS Photos, however, doesn't let me download my photos: > Your full-resolution photos and videos stored in iCloud may not fit on this Mac. I have 45 GB free on internal SSD. And iCloud has at most 5 GB of storage. What is happening?