Ubuntu is based on Debian GNU/Linux distribution, thus it uses Apt package manager.
Default behavior is to keep downloaded software packages on the file system during installation process. During time the packages can use a lot of disk space and we suggest occasionall cache cleaning.
How to clear Apt cache
1) Run file manager (e.g. Nautilus) as root user
2) Navitage to /var/cache/apt/archives
3) Delete all .deb installation package files
Note: Be sure not to delete folder partial and file lock.