Mike,
First of all, to give you an idea. I run a DNN installation that has 5 portals, 2000K users, a few hundred pages and a lot of custom module content and forums. I am using a total of 42mb of database space for that installation, so yes, those numbers are a bit off.
With GoDaddy it is very common for them to not truncate the log file on their routine backups. I would ask them how often they are taking backups and when they truncate your log file. This is something that they should give you an answer on.
Also, to do a bit of digging, run the following command to see your overall database size.
sp_spaceused
See what that equals, then if you have a look at my blog http://www.mitchelsellers.com I have an article on Get all table sizes, where you can investigate the size on disk of all tables in your database.
However, if you .bak file is only 15 mb, I would guess that your total usage is under 20mb and the rest is a bloated transaction log.
if you have any questions feel free to drop me a message as I don't always notice replies here on the forums (msellers@iowacomputergurus.com)