Large/big

[Glen]

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?

[Rusty]

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.