Martin Lindstrom explores it:
[T]he bigger the cart, the more we buy. In fact, if the cart is double the size of our regular one, we buy an astounding 40 percent more than we usually do. It's not as if we need the extra items, but larger carts tap in to our primordial need to hoard food. We're still operating with our primitive brain and, ultimately, we're primed to guard against starvation. Evolutionary speaking, we're hardwired to store food in times of plenty. So, if our shopping cart looks half-empty, we'll fill it.