A grin is a smile, often smaller without teeth showing. The only time I think of a grin as large is if there are modifiers "a big grin" or "a fool's grin". A smirk is often negative in some way. It might be a half-smile. Or a sarcastic smile. Or a know-it-all. Or at someone else's dismay or pain.

A grin is smile, a smirk is a smile with an attitude of over-confidence or pleasure in someone else's failure. Here is a good picture of a smirk http://pre00.deviantart.net/478e/th/pre/f/2008/001/6/d/smirk_by_agrivaine.jpg and another one https://dubsism.files.wordpress.com/2010/11/oj-simpson-smirk.jpg?w=547

Hi Dariana, A smirk is similar to a grin, but the main difference is usually that a smirk is half hidden. It's not hidden enough to not actually be a kind of smile, but it's not a full grin, with the face beaming. A smirk can often be irritating or conceited, and can often be related to gloating over some kind of victory. A grin, on the other hand, is simply when a person is smiling, big and broad across their face with no attempt to hide it at all.

I am wondering, too :))