Um.. Start with checking if the DB folder is big by right clicking > properties? If it is big, then just do the same to all underlying folders to find the cause.
When I had extreme logging enabled the .txt files got quite large
Also you could try and check //Diablo III/Data_D3/PC/MPQs/Cache. It easily gets to 4gb which will eventually add up if you are using several D3's without symlinks. You can delete the Cache folder.
It's probably the log file within demon buddy, everytime you open up db it creates a new log and they stack up a lot. I have almost 1gb worth of logs, you can just delete them if you don't use them.