You can also backup and restore individual tables using phpMyAdmin.
If you are using wassup, that could be your problem. I stopped using it because it is an incredible resource pig. If you are using that plugin and you have not changed the default settings, it keeps building history forever and tables get very large. You can delete the history in the configuration screen.
"It's inexcusable for scientists to torture animals; let them make their experiments on journalists and politicians." -Henrik Ibsen