php composer installing the whole source code and extra files

I'm totally new to composer and read the whole manual and started using it, but there is this thing that I can't figure it out. when I run the command

composer require example/example

ok, composer downloads the source code of library, but it has downloaded the WHOLE source code including the samples, readme, test files, anything. I plan to use many libraries and I don't want extra files to reside on my server.

How can I get rid of extra files composer downloads??