Persistent Subscription checkpoint stream size

Hi - we have a handful of persistent subscriptions that are using the default checkpoint settings, and we have just noticed that each subscription has created a
$persistentsubscription-<Source>::<Group>-checkpoint stream with 10s of thousands of events in it (new events at a rate of 3 a min).

Now I can adjust the checkpoint settings on the subscription to reduce the rate these are added, but my question is. Do we need to keep all the events in this stream - could we set a max age or max length?

Also - should the expiry options not be set by default when the subscription is created?