Several days ago, after posting this question, I surrendered to Apple and ticked the box to synch Photos between my iMac (macOS 12.2.1, 1TB SSD, not Fusion) and my iPhone and MacBook Air via iCloud. "Optimize Mac Storage" was not selected.
I began with synching only to the iMac, not the iPhone or MacBook Air referred to in my previous question. The iPhone synchs to the iMac using the Finder, and is set to download only some of my Albums. I haven't ticked the System Preferences > iCloud box to synch the MacBook Air just yet. Thus, I don't think either the iPhone or the Mac Book Air's storage status is relevant to this question.
Synch took a day or more, and settled down with (as far as I can determine) all items in the iMac's Photos Library copied to iCloud: both the iMac (via the Finder) and iCloud (via icloud.com) report holding the same number of photos and videos.
But the folders' reported sizes are significantly different. On the Mac, System Preferences > iCloud shows 29.86 GB used for "Photos and Videos." On the other hand, the Photo Library itself (Users/David/Pictures/Photos Library) is shown by the Finder's "Get Info" pane as 44 GB.
Why do the sizes of the Mac's folder and iCloud's folder differ?