Looks like it's an issue with this app from the App store:
https://apps.apple.com/app/memory-diag/id748212890?ls=1&mt=12
I use Memory Diag in my menu bar.
As you can see from the requested screenshot, when I hover my cursor over the little Dock icon at the bottom of the Memory Diag dropdown, it is showing my Dock memory usage at 623 MB.
However, Activity Monitor is showing my Dock memory usage at 66.1 MB.
That's a tenfold discrepancy! (circled in red)
So either Memory Diag or Activity Monitor is telling a porky...
Or something else is going on with Dock, because when I hover my cursor over the little Mail and Dropbox icons at the bottom of the Memory Diag dropdown, the memory usage for these apps is almost identical to that given by Activity Monitor. Also, App, Wired, Compressed, etc. are the same, as shown in the screenshot.
Hmmm....
Thanks for your help so far. But the issue with Dock remains this morning (I live in Japan, so our time differences may vary).