Idiom
big of someone
Theme: BEHAVIOR
generous of someone; kind or forgiving of someone. (Sometimes sarcastic.)He gave me some of his apple. That was very big of him.It was big of Sally to come over and apologize like that.
Slang
big of someone
Theme: KINDNESS
mod. nice of someone. (Often sarcastic.)A whole pound. Wow, that is really big of you!Three daisies he gave me! “Oh, that's big of you!” I said, batting my eyes.