Can I delete this hidden folder under /users/.(username)/(username).sparsebundle? It takes up 77GB of harddisk place. Also what is this folder all about?
I recommend against FileVault, frankly. Unless you deal in state secrets, you might want to turn it off. It greatly complicates TimeMachine backups (basically, you can't get at individual files in your backup set easily) and it litters your disk with these sparsebundle files.
Voice of experience here. It seemed like a nifty feature, so I turned it on, then eventually went to the Apple Store to ask a Genius WTF about this very same issue.
It contains the entire contents of your Home folder, which is why it takes up so much space. I don't think the File Vault disk image is any bigger than you Home folder would be otherwise?