I think I've got my arms around this problem. I've actually recovered about 65G of disk space in the process.
Google's version of IMAP combined with Apple Mail is a chatty, noisy, wasteful system. Google creates many folders that replicate a message (or are pointers to the same message). When Apple Mail syncs with Gmail, those folders are created on your hard drive and have actual copies (not pointers) of the messages. Caches are also spawned and the whole bloated mess is constantly syncing and spawning more temp, cache, and envelope files.
A solution is to go into gmail (on the web) look at settings, and then under the Label tab, turn off IMAP syncing with all those extra directories (like All Mail) Just turn them off.
There are two benefits:
It won't expand out of control on your hard drive (1 have .5G on gmail which bloated to 65G on my hard drive)
The constant synching and passing back and forth of files for these various folders will be choked off and you won't risk getting cut off for excessive bandwidth usage.