Hi, @byersoy.
The endpoint you are using is the Get recently created contacts endpoint, so all of the data will be new.
The Get recently updated and created contacts endpoint will return records which have been modified, but you are correct that neither of these endpoints returns only the modified data.
If you only want to receive notifications for changes, the Webhooks API will probably be your best bet. You can subscribe to specific object properties and HubSpot will notify a webhook URL of your choosing whenever an object’s value changes.