I'm not an expert on municipal expenses, but I'd wager that it takes a pretty unusual cost situation to render it cheaper to pave/trench/replace more miles of road/pipe/wire and maintain more service entries to serve fewer people.

This is compex. Rural gravel roads are likely cheaper per capite than paved roads, but they don't scale to the triffic of a suburb. A city street needs to be more expensive yet.

It's not a 'strong statement'. It's a statement supported both by rationality (more inputs spread out across less users) and empiricism -- there's countless case studies arriving at same conclusions. Here's some from Strong Towns: i) https://www.strongtowns.org/journal/2018/8/3/cobb-county-add... ii) https://www.strongtowns.org/journal/2018/9/27/a-texas-sized-...