Deleted message will be moved to Trash folder by default. If you mailbox quota excess, you won't be able to move the deleted messages to Trash and you need to delete mail without saving them to Trash folder. Please refer to the procedure HERE.
Please clear your Trash folder regularly to free up space. Please refer to Webmail user guide section Deleting Messages for details.
You may check quota at https://www.its.hku.hk/documentation/guide/email/quota to determine which mail folder occupied large amount of space first.
Login Webmail, select the mail folder with high disk usage. Click the Size column until the mail sorted by size in descending order.
Please consider deleting the large size mail to free up space. You may also download a copy of a mail before removal. If you want to keep a copy at your local computer, please refer to How to download a mail message in eml format?.