From Jonathan Lermitage:
When you installed a video game from a CD or a DVD, you installed some redistributable packages too : DirectX, Games For Windows Live Redist, VC Redist, Rapture3D, NVidia PhysX Redist, etc. They were installed from the CD/DVD, and those installers didn't stay on your hard drive. The Steam application allows you to download games from the Internet and install them, but what about these redistributable packages ? You probably saw they are installed the first time you launch your game, OK, but what happens to these files when they go wasted ? The answer is very simple : Steam doesn't delete them. An example : install the first complete season of Sam & Max (6 episodes) : every episode is about ~300MB. For each of them, you can count 100MB for the DirectX redistributable package. Yes, it is about a third of the game files size, and they are not removed by Steam. For 6 episodes, you could save 600MB. Imagine what you could save for 10 or 20 games.A little tool that is able to list every redistributable package stored on your Steam directories, and allows you to remove them. This is the goal of TikiOne-Steam-Cleaner. This is a very simple program : set the Steam SteamApps folder (this is a sub-folder of the Steam application), click "Search", you choose what file or folder you want to remove.