That change is much easier once you're into multifamily residential densities, apartments or condos.

Where it's difficult is built-out single-family residential neighborhoods.

If existing code calls for 6,000 square foot lot sizes, and you lower that to 4,000 square foot minimum lot sizes, how do you increase actual density in a built neighborhood?

Someone has to buy up multiple 6,000 square foot lots with existing houses, then combine and re-subdivide the lots.

If you're really lucky, you can turn two 6,000 square foot lots with two houses into three 4,000 square foot lots without having to tear down either of the houses. But in most cases, you can't, so you only really get the 4,000 square foot density if someone can afford to redevelop the neighborhood.

One alternative that's hard-fought in many neighborhoods is accessory dwelling units on existing lots -- adding a mother-in-law apartment or a guest house to an existing home on a larger lot. Regulations usually try to maintain the feel of single-family residential while allowing de-facto multi-family residential.

(It does go the other way, too. My neighborhood was platted as a walkable streetcar suburb, served by the Seattle-Tacoma Interurban Railway. The trolley stopped running before WW-II, but the basic plat map hasn't changed. The older parts of town still have very walkable neighborhoods with homes close to the street.)