i wonder why only 90kB was removed when i downloaded many MB ?
This 90kb might only be the
nginx executable. You can remove configuration files and other files left behind by the program with
apt-get purge <program_name>.
The other MBs you might have downloaded were programs on which nginx might depend on to execute (so called dependencies). To remove dependencies with apt run
For a more canonical answer, consider this:
apt-get remove <package_name> will remove the binary files, but not data or configuration files.
apt-get purge <package_name> will remove everything that has installed, but not its dependencies. This might be useful in case you want to reinstall a piece of software, due to, for example, messed up configurations. You can also do the same thing if you run
apt-get remove --purge <package_name>
apt-get autoremove removes software that was installed as a dependency by some other software, but are no longer needed.