I am successfully using 'defaults write com.apple.menuextra.clock IsAnalog -bool true' to replace to a smaller icon the Apple clock in Big Sur.
Is there a code to revert to Apple's clock if ever required?
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Sign up to join this communityDeleting a key will set it back to its default value.
defaults delete com.apple.menuextra.clock IsAnalog
However, you don't even need to delve into defaults to make this change, to or from analog — it's in the UI: System Preferences → Dock & Menu Bar → Clock → Time Options.