For the purposes of this specific conversation, the layout of typical Japanese supermarkets, the cost of groceries and the frequent lack of specialized parking for supermarkets, Japan is probably an irrelevant comparison. Where there are parking lots, people typically purchase only what could be carried back to their car without carts. Bicycles are used for shopping with much greater frequency than countries like the U.S. Shopping carts are typically taken and returned at the entrance of the store before the customer exits. At Uwajimaya in the United States (a Japanese asian market with stores in Oregon and Washington), remarkably, the same cultural use of shopping carts occurs.
Most people in Japan live outside of the Yamanote circle in Tokyo. Rural and Suburban supermarkets have parking lots (although in central areas they can still be quite small) and people still use cars for shopping trips, especially in the countryside.
It is true that grocery packages are much smaller than the US (since Japanese houses, even in the countryside, are smaller and I guess the average household size is smaller as wel). Shopping carts in regular supermarkets are smaller than abroad, and are usually built to house 1 or 2 shopping baskets you can also carry by hand.
But hey, we still have Costco in Japan, and package sizes and shopping cart sizes are just as big as they are in the US (although the parking lot is probably considerably more crowded). And Costco is extremely popular here. It's far messier than a Japanese supermarket and I do see inconsiderate people sometimes in Costco, but the cars are still parked nicely and most people do return their shopping carts. It would be interesting to compare Costcos in Japan and the US directly though.
I lived well outside of Tokyo and most supermarkets were eki-mae (near train stations) and did not have parking lots. I did go to markets by car, and the practice I mentioned, shopping carts used exclusively within the store, was practiced. There very well may be exceptions, but as I noted, the cultural practice even extends to Japanese markets in the United States.
