well, it was part of her plan to get those people on her side. Instead of saying that she has a big heart, I think saying that she's 'clever' suits her more. I think that statement fits psyche more.
(just my opinion tho)
From a bigger picture, I don't think Medea is as twisted as she thinks she is. I feel even though she may clever/manipulative in winning people's hearts, she's also wanting to save them deep down from the injustices that she has faced that has befallen onto them too. That's just my speculation on her character. Psyche is kind and sweet but I don't think she's big hearted in the way I see Medea. Pysche can only think of the momentarily big heartedness to people but Medea thinks about the over all encompass of big heartedness for people.
Her heart is bigger than any ocean. I teared up when the Butler realized how special she was. It's a same how bed of a reputation she has.