Expansive is all about being able to grow or get larger. An expansive piece of land is large, an expansive mind is one that is always thinking of the big picture, an expansive vocabulary is one that holds lots of words.
Warning: don't confuse expansive, "large," with expensive, "high priced." They differ by only one letter but mean very different things. A Big Mac may contain what McDonald's advertizes as an expansive all-beef patty, but it isn't expensive food.