strange virtual disk size - please help

Hi, I really need help with the following.

I am using a Mac Book Pro on which I am running Ubuntu with VirtualBox.
I set the virtual disk size to 40Gb dynamically allocated. After a couple of weeks the virtual disk size limit of 40Gb is reached.
My / folder contains only 8,4 Gb of Data. So whats taking all that space???

Thanks for your help.
Misha