Changing Spark Plugs... Am I missing something here????

I went to change the spark plugs on my 98 4.6L and couldn't get a spark plug socket that would fit down the hole. The socket was simply too large for the diameter of the hole. I went to a local auto parts store and could find a socket with a smaller outside diameter. Am I have a brain fart here or something? I'm sure its my stupid fault but can someone point me in the right direction here....