Difference between mail account size and usage limit

Difference between mail account size and usage limit

Hi,

We have and professional account and we notice something that's a little confusing.

There's an account which has about: 386MB on Account Size (Settings > Mail Accounts).

But when admin goes to usage it shows: 78% of 5120MB (Control Panel > Mail Accounts > user@domain)

What the difference between this numbers (Size and Usage)?
Why size changes for each account?

Thanks!

Best Regards

Anderson Costa