What does the sentence mean?

It means she has to look at the whole situation, not just some small part of it, and consider how whatever it is he's talking about fits in with their whole lives and what's happening in the world at large.

Nancy, have you ever completed a jigsaw puzzle? Individually, the little pieces don’t necessarily reveal much, but once you put all the pieces together, you can see the “big picture.”

In your example, I think this guy’s “big picture” (what he wants) might include money…wealth. I surmise that based on his comment about grabbing the biggest slice of the pie.