If the disk usage is higher than you think it should be, or just want to know how DA is coming to the disk usage value, you can type:
cd /usr/local/directadmin echo "action=tally&value=username&type=user" >> data/task.queue ./dataskq d800
Where username is the name of the user in question. This will run the tally for that account and dump out all the bits that are added together to come to the final value (system quota, email, database, etc.).