Greenby mailbox sizes - no resemblance to actual usage

[#1]

I shall be moving my and my wife's emails to other providers soon so this post is more to satisfy my curiosity than anything else. We have three mailboxes with Greenby that have been recently moved from Plusnet. Two, in my name, are empty (including spam and trash). My wife has about 20mb of emails. Greenby tells me her mailbox is using 316mb! At this rate we would hit the 2gb limit in a month or so.  Deleting emails and removing from trash makes no difference to the amount of space that the mailbox is apparently taking up.

I raised this question on Greenby and got an AI answer that failed to understand the problem, saying smugly that the amount in use was not large in comparison to the total allowed.

I wonder if others are seeing stupid usage figures for their mailboxes and if so, how they are dealing with the problem before they run up against the max allowed.

[#2]

Are you using IMAP OR POP3?

Using POP, with it set to delete from server on download, all my mailboxes are showing 0% used.

[#3]

@anthonyg 

Check the WebMail view of the storage usage for the all the folders.

The historic Sent Items can be where quite a bit of extra storage is being used.

For my email a/c the usage figure is slowly rising, it is at about 10% of the actual.

[#4]

I am using webmail to review the mailbox. Sent items is irrelevant because my wife has been unable to send any emails using her email clients and she does not use webmail (but I looked anyway and there is nothing there).

The issue is that the emails actually in the mailbox, in all folders, in no way resemble the amount of space Greenby tell me is being used.