Big picture

It doesn't really mean anything but a polite way to say:"I don't know how to reason with you right now; just drop whatever you are doing. This is my show."

The reasons that Chinese bureaucrats are saying that all the time are: 1) they really have no idea what else to say; 2) it doesn't take much to get them out of the embarrassing situation of being questioned; 3) and at the same time, make you feel like an idiot, because you are not thinking intelligently, effectively preventing you from questioning further.

If you are really dumb enough to pursue the meaning of the "big picture", chances are those bureaucrats would start yelling "you are naive" at you, putting up an air that they are truly insulted and won’t even honor your question with an answer.