What reasons do girls give a guy a big cheesy smile?

I finally had the guts to talk to this girl at my college that I like, I didn't really flirt or anything. She did keep the convo going a tad bit when it could've ended. Really wasn't long though because we had to get to class, but she seemed really attentive to what I was saying and held solid eye contact.

After she got done saying what she was saying, she just grinned at me, and I said " well alright then, have a good one and waves " and she had this wide cheesy smile that did reach her eyes and held eye contact as I was walking off and waved back.

Now could this have just been polite or was this a possible decent sign? Not bothered by my presence?

What reasons do girls give a guy a big cheesy smile?
Post Opinion