I have studied the movement of Q2A database in this matter. When you move blob data to disk, blob contents in database will be empty. But, depending on the server type and configuration, MySQL may not release the space that was reserved. And, your database might be a bit huge in other causes (e.g. index for search, etc). Try operation below after backup your DB.
Optimize all tables with phpMyAdmin
"Admin" > "Stats" > "Reindex content"
I have not seen your database, my advice may not be effective.