Here, “big” means important or serious. 重要。 Examples: “I’m having a big meeting today with my boss.” The meeting may be small in size (just you and your boss), but the meeting is “big” because it is important to you. Maybe you are asking for a raise in salary! Another example: “The President’s decision to suspend the trade agreement was a big deal.” “Big deal” is an idiomatic expression meaning “an important event.”