What I mean by “big picture” when it comes to world-building is your world outside your “small picture world.” There. That explained it perfectly, right?
OK, I’ll give a couple examples to make it clearer. One story I wrote recently takes place in an alien jungle (a kid and his uncle get lost in the jungle and need to escape it). I spent so much time fleshing out the jungle and the creatures in it (small picture world-building) that I totally disregarded the big picture world, such as: What defines this new age of the future? How did this age come to exist (i.e., how did humans on Earth evolve to the point where they could send spaceships to other planets and what not)? In light of this new age and the changes that had to happen in order to get to this point, what are the fears and values of this society? Etc., etc.
I’ll give another example. I also recently wrote a story (an epic fantasy) that consisted of two neighboring nations that’d been at war with each other for generations. One is made up of people who can telepathically control animals, whereas another is a group of technologically gifted people (an electric current runs through each of them) that have formed a city full of metal structures and complicated machinery (the people power the machines with the natural currents in their bodies). I spent so much time brainstorming and ironing out how these two different societies function that I, again, forgot to consider the big-picture world (I did later, however), such as: What is the rest of the world like? Are there other societies of different types of people? If so, how do they affect these two groups/nations? How did these two groups develop such different abilities based on the world they live in? Etc., etc.
So, moral of story: It doesn’t matter when you do it, but make sure you set aside time to brainstorm your “big-picture” world if you haven’t already. Then go and incorporate it into the story. That way, your world (and story) has more of a complex, fully-fleshed feel. Go on, try it. Think big, class. Think BIG.