My 2005 Prius comfortably fits me, my wife, our two kids in car seats, and luggage for a weekend. We can't bring our dog, because her crate takes up too much room. Our gear usually consists of one medium-sized suitcase, two laptop bags, diaper bag, and a bag of toys, a bag of food, and maybe a small cooler. We're able to pack all of this, without obscuring the rear window. A week-long trip would require more stuff, and I'd have to block the rear window, which I hate.

As others have said, packing a smaller car requires you to make more decisions on what you bring. Do you need a stroller, when you have a Baby Bjorn? Do you need all those toys, when there's probably plenty to entertain the kids at the destination? Do you need a pack & play, when your kid could sleep with you? (We keep one at my in-law's beach house, so we don't have to pack it.)

I had a girlfriend, who's mother worked for British Airways. Every year, they'd travel to Australia, for a week. The mother, father, and two daughters would each bring one backpack, with everything they "needed" for the week. They'd decided they'd rather do laundry, on vacation, than bring seven changes of clothing. I try to keep them in mind, whenever I travel.