library/mail folder storage very large

Background: Using an IMAP account, I switched from Outlook Express (under VM Ware on same MacBook) to Apple Mail because I wanted better archive control (generic .mbox versus proprietary MS .dbx archive format) and to get past the OE 2gb message file limitation. As an Attorney, I download every message and attachment to local drive and backup and archive.

Q1: Gmail says I am using 6.1gb out of 7.6gb with 8,146 "All Mail" messages. Apple Mail "Account Info" says I am using 6.1gb out of 7.4gb (close enough), BUT counts 14,392 messages at the same time. Why the difference in message counts?

Q2: "Get Info" for Users/.../library/Mail reports 13.24 gb, over DOUBLE the actual storage for all the messages and attachments. Why the increased hard drive space?

Q3: Mail Preferences "Accounts/Advanced" has a setting for "Compact mailboxes automatically" which is selected and greyed out so that I cannot change it. In theory this sounds good, BUT I am finding that Apple's documentation for Mail is very poor and imprecise, and I do not know what Apple "compact" actually does. In other email programs, "compact" can actually strip the message and leave only the tracking and Subject data, which I do not want to do.

What is actually happening in "compact", and why can't I changed it?

NOTE: when I setup Apple Mail, I was not sure exactly what some of the Preferences should be to get the same end result as with Outlook Express. I told Mail to sync with Gmail and let it download ALL messages and attachments, which I think it did. When I noticed the message count discrepency, I checked to see if anything was missing, but found not evidence of loss. I did a quick lookup on "Rebuild" and gave it a try. It took all night at 3megs DSL download speed and it appears from the data storage size that everything is downloaded, even thought the message counts differ. I do not know if "Rebuild" first makes a "Copy" and then duplicates the message store, and then deletes the "Copy" as cleanup, OR if it duplicates the existing data files. I have not found any duplication in the files or in the messages, but this may explain the doubling from 6.1gb to 13.24gb. Any ideas what is happening?