By default, Thunderbird is configured to synchronize (download and store them on your computer's storage for offline access) ALL messages in ALL folders for IMAP accounts. This is responsible for high disk space usage in your case, but you can change that behaviour by changing a few settings in Thunderbird as follows:

In the Folder Pane (the area showing a list of your accounts and folders in Thunderbird), right-click your Gmail account and select Settings. This will open the Account Settings tab with your Gmail account selected. In the left pane of this tab, underneath your selected Gmail account, click the Synchronisation & Storage option to load that section of your Gmail account's settings. At the very top of this section, there is a checkbox labelled Keep messages in all folders for this account on this computer. Uncheck that option. This tells Thunderbird to NOT download and store them on your computer's storage for offline access, but instead, only download message headers from the server. The body of the messages will only be downloaded on demand, for example, when you open them.

The next step is to claim disk space that's currently being used for storing messages that Tbird had already downloaded up until this point. This basically means deleting all local files and folders. Begin by opening your profile folder. The quickest way of doing so is to go to Thunderbird's Help > More Troubleshooting Information menu. In the tab that opens, look under the Application Basics section, in the row labelled Profile Folder, click the Open Folder button to open your profile folder via Windows File Explorer. Quit Thunderbird now.

In your profile folder, you will see a subfolder called ImapMail. This is where Thunderbird is storing data for your IMAP accounts. Open it and inside you'll find yet another folder with Gmail's IMAP server's name, e.g imap.gmail.com. Open that folder and inside you'll find several files and at least one folder. The big files that have no filename extensions are MBOX files, where Thunderbird saves downloaded messages. You'll also find MSF files that match the big MBOX files. Each MBOX file will have a corresponding MSF file. You may also find folders that have a .sbd suffix. These are what you see as subfolders in Thunderbird if you have subfolders in your account. Finally, you will see a filterlog file and a msgFilterRules.dat file. Delete ALL .sbd folders, MSF files and the large MBOX files. You should end up with nothing except the filterlog and msgFilterRules.dat files.

Start Thunderbird. It will recreate the deleted .sbd folders and MSF files, but it won't recreate the massive MBOX files. It will fetch ALL headers from Gmail pretty quickly because the headers are very small in size. This is how things will remain going forward unless you further change Synchronisation & Storage settings for this account in Thunderbird. For example, you may wish to allow Thunderbird to synchronize some INBOX messages by age, for example, the most recent 1 week old or 1 month old messages. You can also limit downloads by size of the messages. In such a case, Thunderbird will recreate the INBOX mbox file, but it won't grow as big as the deleted one was, because Thunderbird will only download and save the messages that meet the age criteria or size that you've specified. This is a nice balance between saving disk space and keeping offline access to your newest messages. Nothing will be deleted from the server by any of the actions performed above.

The reason your profile is twice the size might be because the All Mail folder is subscribed. Most users unsubscribe it since it duplicates mail in all other folders. See the All Mail section here.

The problem of constantly downloading IMAP folders has been reported by some users, although I don't see it myself with a gmail account. It could be due to folder corruption caused by incomplete compacting or antivirus apps. Try adding the account to a new profile (Help/Troubleshooting, about:profiles), and see if the constant downloading persists.