When you install something in Ubuntu, package files are downloaded for installation but are not automatically deleted after installation is over. Instead the files will be used as a backup in case the application needs a reinstall in the future. But in most cases, this is a sheer waste of space especially if you are using a old laptop or a netbook with minimal amount of disk space.
http://www.techdrivein.com/2010/09/free-up-insane-amount-of-disk-space-in.htmlTwas only taking up 90MB in my case...