Large/big

Large/big
 

I'm having trouble explaining to a friend why he ought to tell the clerk he would like a large coffee rather than a big coffee, since he rightly thinks of both terms as meaning grande. I tried saying that large is used in comparison with the other available sizes of medium and small, while big is a more general term, but somehow it didn't sink in with him - I'm not even sure of it myself! Is there a good way to distinguish these two, since they both mean the same thing in Spanish?

Teach your friend about locutions. Certain words are acceptable when used in conjunction with other words while other words, even if we substitute what may be considered a synonym, are not acceptable.