you can download and install windirstat
. it will tell you specifically what files are in your installation as well as how large the files are. it gives a graphical display of those files, as well, so you can really see what files are in your installation and the size of them. the larger the block, the bigger the file.
the first pic shows the program querying your system and the second pic shows the graphical representation of the files. i have team fortress 2 content.gcf
highlighted and there is a white square on the bottom that corresponds to the size of that file within files>steamapps>steam>program files (x86)>windows(c)