You’re getting that sync failure on your Mac because you’re using an updated version of the app that moved that check earlier in the syncing process. If you’re using an older version of the app on your phone, it doesn’t have that new code yet.
The error is still correct though – your collection is too big to be syncing with AnkiWeb.
Where to look for the problem, and what to do when you find it: Constant Errors Uploading / Sync - #4 by Danika_Dakika .