It depends on zoning laws, and how they were built, and how much a developer can get.
Some developments, especially in cookie cutter houses, where there are only 4 or 5 different models, came with a garage, and a driveway that can park another car on it or a little more. The houses in these neighborhoods tend to be very close to each other, no more than 10 feet, so they make up for it with a bigger front yard. Because they can fit in more houses in these tract developments, the backyards sometimes tend to be smaller.
There is also another reason - as you get older, many people do not want to deal with a large backyard, and all of the gardening and maintenance that goes with it.
Now, that depends on how much you pay for the land and house. I have seen many different ways, especially in the suburbs, etc.