Software updates in Ubuntu is very often.Update Manager need to download available packages into local repository before install them.Those packages take the space and become larger and larger if you don’t delete them after installation.
By default,you can delete them manually from /var/cache/apt/archives file folder.Here is a small tip about how to auto-delete these downloaded packages after installation.

First,open up Synaptic Package Manager from System->Administration menu.

Navigate to Settings->Preferences,click Files tab and check “Delete downloaded packages after installation”

Next time you update software through Update Manager,the downloaded packages will be removed automatically.