My Samsung Galaxy S8 is using tons of data from Google Play Services. We've stayed within our data forever, but suddenly this is using way more than ever. How can this be?
Your concerns about Google Play Services using more data than normal are definitely valid. A number of things can cause higher than normal data usage. As bearone21 mentioned, downloading and updating apps can cause more usage to display for Google Play Services. Mama23dogs' suggestion to make sure Google Play is set to update your apps over Wi-Fi only is a great step to take. This can be done by going to Google Play > Menu(three lines upper left) > Settings > Auto-update apps > Over Wi-Fi only. Choosing this setting for App download preference is a good idea too. Try those and see if you notice a difference.
We still want to make sure there's nothing else going on though. How long have you noticed the increased data usage from Google Play Services? What changes were made to your device prior to the increase in data usage? What software version do you have installed on your phone? Confirm this information by going to the main phone Settings > About phone > Software information > Build number.