So, buildings here tend to fill their lots and not have yards. They're certainly *allowed* to have setbacks and yards, unlike the draconian land-use and FAR (floor area ratio) regulations of the US, but through much of Osaka they don't. (There are yards in Japan, I've seen them in Nara and Kyoto away from the city centers, in Kyoto not even that far from a train station, in Nara not far from a bus running every 4 minutes.)
Imagine that every lot is 1000 square feet, which allows for a quite ample two-story house, even with a parking space or two (say 200 square feet per space), and/or a strip for plants. Imagine that half the urban land is devoted to such residential lots (after streets and non-residential uses.) That allows for 5381 houses per square kilometer. Assuming an average of 2 people per household (2.55 seems a more accurate 2010 number for Japan) that's nearly 11,000 people per square kilometer -- considerably denser than San Francisco or Somerville (both around 7,000) or anywhere else in the US outside NYC. At 2.5 people per house that's 13,500 people per square kilometer, on the order of Bronx and Brooklyn. Without needing a single home taller than 2 stories, and giving 1000-2000 square feet per home (unless you build a one story home with two parking spaces, and then you're just asking for it.)
It certainly can be nice to have your own yard. But US yard are big enough for second homes. We shouldn't be *requiring* them.
(Note: Osaka overall doesn't look like this, there are many tall buildings. Parts of it and I think Tokyo do look like it, though. And it's an interesting exercise. And my current lot is probably more like 200 square feet.)
 Interesting effect of most of the streets being one-lane alleys shared by all modes: no sidewalk, so no curb cut effect from having a driveway.
Parking lots and garages in the US need at least 330 square feet per car because of access lanes, but curbside spaces or house parking that opens directly to the street are different. Hmm, actually the space use of driveways should include the curb cut and denied parking space as well as the car space on the private lot, but again not an issue when there is no curb or street parking.
See the DW comments at https://mindstalk.dreamwidth.org/531875.html#comments