Fat chance versus Slim chance!

thanks to my friend, I came to know that the phrases "fat chance" and "slim chance" mean the SAME!! it seems that, people use "fat chance" in a sarcastic way, when they mean that there is less or no chance at all! A common analogy seems to be the usage of "Big Deal" ? [1]

Also, a search for "fat chance" in google brings up lot of such usages...