quick question, PLEASE HELP

whenever I download a file with the extension ( .sit ) all it does is open up a blank microsoft word document. My default for opening downloads is stuffit expander, but still, i dont understand why I cant read ( .sit ) files. It took about 10 minutes to download the 130MB file, yet when it opens...its only a blank word document. I know that there is no way that a simple word document could be 130MB. Is there something that I;m not seeing? please help

If it means anything, I am trying to download the newly released version of Bryce 5.5 for free! go to downloads.com and try it out for youself and see if anyone has the same problem.
thanks

You need to change the default application for your .sit files.
Select the file and hit Command + I to open up your info window.
Where it says "Open with", make sure it says Stuffit Expander.
Then hit the "Change All" button.