Why is my email inbox full even though I have deleted emails?

Make sure you are deleting emails from the server.
Delete emails from your inbox only will not necessarily remove it from the server and, subsequently, from your available storage space. You can check your email client settings to ensure emails are being deleted from the server after you delete them from your inbox.
Check for mailing lists.
Subscribing to newsletters and mailing lists can take up a lot of unnecessary storage space if you are not actively following them. Unsubscribing from these lists can save a represented amount of storage space in your inbox.
Check for large files.
Attachments sent to your inbox can take up an odd amount of storage space. Take a look at the emails with the largest file sizes to gauge which emails are taking up the most space.
Delete unnecessary emails.
After managing your large files, you can further your inbox cleaning by managing your content. Deleting emails older than 60 days on a consistent basis can also help with clearing unwanted storage.