I'm not sure if I understand mailbox size limit and caching mode correct.

We found that some of our users use caching mode to circumvent mailbox size
limits. We don't have "Limits apply to Cache" set but are considering this.

Help for this setting says: "If you impose this limit on users who have
existing Caching mailboxes, their Caching mailboxes might be reduced in size
in order to meet the new disk space limit. Such users should be warned in
advance so that they can back up their Caching mailboxes before the size
reduction takes place. Otherwise, users could lose messages that they want
to keep."

I'm confusued about this warning, because I was not aware that Caching
mailboxes can grow bigger than online. My perception was that the only
restriction of "full" mailboxes was that users are not able to send mails -
but no mail will get lost. If so, caching mailboxes should always contain a
copy of what's on the server - and vice versa?

Now thinking about it, the only idea that comes to my mind, that might
explain the difference is the "Expire" action in Mailbox Maintenance; maybe
this affects only the online mailbox?

My question: We don't use "Expire" - will in this case caching and online
mailbox the same?