Limit OST PST File Sizes
So MS doesn't seem to understand the concept of what cache is. When they mention cache what they really mean is full replication.
Or as MS states
"The format that Outlook uses to store data locally for Cached Exchange Mode is less space-efficient than the server data file format. This means that more disk space is used when mailboxes are downloaded to provide a local copy for Cached Exchange Mode."
Not only that, but if hit the capacity of the cache you are no longer able to download new messages. You would normally think that cache would simply keep the most recently used data locally and delete out the least accessed data to make room for the new data. If you don't want your users to deal with a pop-up you have to enable AutoArchive that dumps old emails into yet another local file (PST). So now you have to double down on what MS says is inefficient data files.