Apple Mail Archive Help

[Seedouble]

Hi there. Here is my problem:

I am wanting to archive my mail so i can store it on my external HDD and clear out a heap of messages stored on gmail (i'm at 98% capacity, which is about 7GB in total). I have my account set up as IMAP.

I figure later I can just search through the archive in finder or temporarily re-import them if I need to get hold of some info.

Now, I thought: since all my mail has been downloaded already, I can just right click on the [gmail] folder which contains it all, and select archive mailbox. It looks like it will do it eventually, but it appears to be downloading a whole lot of what I suppose are attachments.

Is there a way i can get around this? Do I have the wrong idea and stuff must be re downloaded? Is there a better option?