Large housing projects require less material and construction than low density housing. Why? Because low density housing is essentially doing a relatively giant housing project per person, spread over way more physical distance. People have to live somewhere, so if high density housing isn't made then they have to live in energy and resource wasting low-density housing.
That being said, living in high density will always be more energy efficient as less energy is wasted on locomotion for people, less energy is needed to service garbage pickup/disposal, less electricity is needed for heating/cooling, less material to construct a habitable unit, less energy is wasted transporting people to their work, less energy is wasted getting food to people (one grocery store to serve a whole high density area rather than a large distributed network of grocery stores), less energy wasted on food delivery (delivery driver can just go to one building versus driving the car around the suburbs)... and the list goes on.
Large housing projects require less material and construction than low density housing. Why? Because low density housing is essentially doing a relatively giant housing project per person, spread over way more physical distance. People have to live somewhere, so if high density housing isn't made then they have to live in energy and resource wasting low-density housing.
That being said, living in high density will always be more energy efficient as less energy is wasted on locomotion for people, less energy is needed to service garbage pickup/disposal, less electricity is needed for heating/cooling, less material to construct a habitable unit, less energy is wasted transporting people to their work, less energy is wasted getting food to people (one grocery store to serve a whole high density area rather than a large distributed network of grocery stores), less energy wasted on food delivery (delivery driver can just go to one building versus driving the car around the suburbs)... and the list goes on.