Yes, that's exactly it!

> or because every individual is getting less and less of an overburdened commons?

No, it's not that at all. Why would common services be overburdened? Everyone still gets their water, sewage, electricity, internet, etc., but it's far cheaper to provide per-person.

And with the density you get to build public transit, so people aren't burdened by having to necessarily own a car.

If you had to pay the real bill for road maintenance alone suburbs would no longer be viable.

So the suburbs take from the commons and don't give back in your example.