Your Mailbox Storage is the total size of data occupied by all your email accounts including the size of POP accounts configured in Zoho Mail.
The numbers in the Size column, reflects your current storage for that particular account.
For Business Users who have fixed storage limits, when your Mailbox storage exceeds the quota allocated for your mailbox, you will receive a notification. You can delete some messages and create some free space. The best way to clear up mailbox to create space is to remove any unnecessary POP accounts, or empty unwanted folders.
You need to empty Trash and Spam for the changes to be reflected in the Mail Accounts page.
Further, if you need to have a back up of emails in your Zoho account, you may download it to a POP client. The configuration details for configuring your Zoho based account over POP is available in this help page. The only option available to back up emails in the POP accounts added to Zoho Mail is the 'Export email' option. There is a restriction that you can download only the past 1 year's email and not older than that.
You can also choose to upgrade your account in the Control Panel.
Please write to us at support (at) zohomail (dot) com, in case you need any further details in this regard.