In the Exchange Organisation, there are quotas per Public Folder mailbox for the items ...
..and as a whole organisation:
If you receiving warnings about quota when using public folder, then you need to change organisation wide settings. For this example we have had DefaultPublicFolderIssueWarningQuota set to 1.7GB and DefaultPublicFolderProhibitPostQuota set to 2GB.
Now, we change it to 35 and 40GB respectively:
And all is set now!