Contacts do not sync from iCloud to Mac properly, if at all

My contacts are synced to iCloud on my phone and on my Mac.

I log in to iCloud through the web and I see a full list of contacts with groups

I look at the contacts on my iPhone X (iOS 11.1.2) and I see a full list of contacts, with groups

I look at the contacts on my Mac and I see a tiny subset of my list of contacts - 39 contacts and no groups. This is new, though I cannot say whether it has "never been right since [an action]"

All help and pointers to get the full list of contacts onto my Mac will be gratefully received. Obviously I do not want to take an action which will sync the subset with iCloud and thus replace my 203 real contacts with my 39 subset.

Please feel free to explain from first principles! Treating me as if I know nothing will work far better than assuming any expertise on my part