IMAP: deleted & sent duplicate and reappear

I'm using Apple Mail with five different email accounts. I desperately need to free space. I created a smart mailbox where I sort all the messages by size and I discover I had hundreds of messages with attachments ranging from 1 MB to 80 MB.

- In many of them I removed the attachments, in others I simply erased them, until all my messages had only 1 MB attachments or smaller.

- A day later I've noticed that all my messages are still there. They reappeared.

When I went to all the folders in one of the accounts (a Gmail account called "Gmail") I've noticed that I have the following folders:

1 - [Gmail]

---> (subfolder) Sent Mail

---> (subfolder) Trash

---> (plus others, drafts, etc)

2 - Deleted Messages

3 - Deleted Messages (Gmail's user)

4 - Trash

5 - Sent

6 - Sent Messages (Gmail)

7 - Sent Messages (Gmail's user)

I didn't create any of these folders or subfolders. So, in other words, my deleted messages and sent messages are duplicated, triplicated, in my mac, on the Gmail server, etc. I don't know how to get rid of them, what folder to erase what not, what is system created, etc.

Any recommendation?