Noble or commendable. This phrase is often said sarcastically. Wow, you're actually going to spend time with your brother tonight? That's big of you.It's big of you to reconcile with people who have wronged you in the past.
big of one
Generous of one, as in It was big of Bill to give his brother his entire paycheck. This expression may be used either straightforwardly (as above) or sarcastically, as in How big of you to save the absolute worst seat for me. [c. 1940]
big of someone
1. mod. magnanimous of someone. It was big of Tom to come back and apologize.
2. mod. nice of someone. (Often sarcastic.) A whole pound. Wow, that is really big of you!