For example - if you live in a commie-block - you don't need and often you don't have the option to own multiple cars per family. Often you don't own a car at all. Parking space is at premium and walking/public transport solves most of your transporting needs anyway.
So that's the difference between producing 1.75 cars per family vs let's say 0.75 in the city. And then driving let's say 5000 km per year vs let's say 30 000 km per year. And changing these cars every 3 years vs every 6 years.
Heating is another thing. Surface area of my block is maybe 4 times the surface area of a detached house. But it houses 16 times more people.
But that's nitpicking. The big thing is infrastructure.
How many km of roads, cables, pipes, etc do you need per 1 detached house vs 1 flat in a commie block? How much concrete and steel do you use for that? How much energy is spent building and maintaining that infrastructure over the years?
My 4-stories commie block houses 16 families in a plot that would be roughly equivalent to a plot where 1 detached house would stand in the countryside. And it's pretty low-density as far as city planning goes. You could easily make it much denser. There's a small park and a playground in that plot of land.
So that's ~1/16th the infrastructure to service the same number of people compared to the countryside. And building the infrastructure is cheap compared to maintaining it for decades. The real unsustainable part of suburbia is maintaining all the extra infrastructure that low population density housing requires.
If anything - 1 order of magnitude was an underestimate.