In their example they're buying groceries on a roughly two-week schedule. A family of four or so can go through a lot of food in two weeks. In college I'd get groceries about weekly and my roommate and I would fill the trunk of an Accord with groceries.

Fill is an interesting verb. I suppose you could try to tetris it up each trip to the store, but unpacking would just as inconvenient as packing. That said, if you buy a weeksworth of food for 3 kids, sedan may easily overflow.