To grin from ear to ear is idiomatic, the smile on a person's face when feeling elation.
to smile a very wide, beaming smile.
She was grinning from ear to ear as she accepted the prize.
We knew Timmy was happy because he was grinning from ear to ear.
grin/smile from ear to ear
to look extremely happy (usually in continuous tenses)
We've had a fantastic response,' he said, grinning from ear to ear.
grin from ear to ear also smile from ear to ear
to give a very big smile
He was grinning from ear to ear, as if he had just won the lottery.
Just remember that it's that 'big' smile when a person feels extremely delighted or happy.
No, "grin" means:
To smile broadly, often baring the teeth, as in amusement, glee, embarrassment, or other strong emotion.
"Grinning from ear to ear" simply means that you have a wide smile on your face, usually out of amusement or satisfaction. Usually an ulterior reason behind the smile will give it a reason, say for example,
you sedated your neighbor's dog, the one that barks all night long.