Looks like it tries to download every email in Gmail and index it locally. God. My maillist subscription just made my HDD explode. Didn't IMAP provide some sort of search function already?
IMAP does provide a search function but it's substantially slower than downloading and indexing your emails locally..
I quite like the iPhone's compromise: it stores and indexes the most recent 50 or so messages, and when searching provides a "Continue search on server" button. Thunderbird (or others) could have a larger store of recent messages, but this would avoid downloading thousands of mailing list emails.
You can easily configure how many months of email you want TB to download for you. Go to Account Settings.