From Longman Dictionary of Contemporary Englishit is big of somebody to do somethingit is big of somebody to do somethingspoken a) KINDused to say that someone is very kind or generous to do something b) UNKINDused when you really think that someone is not kind or helpful at all £5! That was big of her! → big
Examples from the Corpus
it is big of somebody to do something• A whole dollar! Gee, that was very big of her!• I think it was really big of Larry to admit that he made the wrong choice. 