The actual mind-blowing scale is that Apple's push notification service isn't just carrying iMessages. It's also carrying push notifications for every third-party messaging app.
And the non-messaging apps with notifications too.
And the silent internal notifications. You added a meeting to your calendar on your Mac? Push notification to your iPhone to tell it that the iCloud data changed and it needs to update. Changed a file on iCloud Drive? Push notification to sync your other devices. Got a phone call, and it starts ringing on your Mac too via Continuity? Push notification (encrypted like an iMessage).
Just how many messages are going through that service every second?!