Until the middle 1960s, the terms were used interchangeably by planners, often to disastrous effect.
In Chapter 5 of Space and Place
, Tuan discusses at length how what is spacious and what is crowded are relativistic concepts, relying almost entirely, in his conception, on the perception of the viewer. This is particularly important in a judgmental sense, as crowdedness has strong implications of being a negative experience, with spaciousness being the opposite. He utilizes the example of a stadium as an objectively densely packed environment that is not only pleasant, but can seem spacious, comparing it to a highway where even a modicum of crowding can make a driver feel stuck and contained.
This line of thinking is very similar to that taken by Jane Jacobs in The Death and Life of Great American Cities. At the time, the doctrine of most urban planners generally stated that any population density above a certain number, by necessity, was overcrowded at best, and a slum at worst. Yet many of the neighborhoods Jacobs was studying, neighborhoods she cherished, were derided under this criterion even though they excelled in many of the other health and well being criteria the same planners had been using. The example she used was Boston's North End, which though primarily working- and lower middle class, a neighborhood of mainly flats rather than individual home ownership, was succeeding in almost every objective and subjective measure she could apply. She brings up the North End with a Boston planner who quickly derides it; claiming it will have to be torn down and rebuilt much like Boston's West End had been. She famously responded that he should wish all Boston neighborhoods should come across so well.
Her point, one she defends valiantly, is that overcrowding and density are not one in the same thing. Many people can live in a relatively small area without that area being overcrowded or unhealthy. Planners, not only from a cultural drive towards suburban ideals but from a desire to be scientific, had attempted to quantify overcrowding in terms of persons per acre not accounting for, for example, the number of people per room, the square feet per person, or (perhaps most importantly) the desires of the individuals concerned. Jacobs frames her argument in the terms that not only can density be a good thing, but he very idea of persons per acre is misguided; instead she prefers the terms of persons per room or individual dwelling.
Tuan goes even farther, claiming that perceptions of what is acceptable are highly culturally and individually determined. He uses people like the Bushmen to indicate a group that lives in very small spaces by choice. Tuan also uses the example of many working and lower class American (or immigrant) families who enjoy living in close proximity.
The arguments neatly come together: traditional planners, indebted as they were to suburban ideals, could only see density as overcrowding. Jacobs, a lover of city living, points out that not only is that not true, it is objectively false, as most neighborhoods not in abject poverty had a certain amount of square feet per person. Tuan demonstrates (if his anthropological references are correct) that even moreso than Jacobs, the question of what is enough space varies dramatically from culture to culture, individual to individual. In this conception, no number can be assigned to what is too dense, or what is too empty.
This does bring up a difficult. Without suggesting a specific number of persons per foot, persons per room, or persons per dwelling, Jacobs does suggest that there is a limit below which an area can be considered overcrowded. She does not desire to recreate the famous slums (of course, a very loaded term) of East coast, American cities. And this is understandable, seeing as how Jacobs was writing for a primarily American, and to a lesser extant Western European, audience, from an American cultural viewpoint. However, even for the Bushmen of Tuan, I am sympathetic to the idea that there is a minimum space per person that can be considered healthy or well, even if that number is difficult to quantify. For example, the Bushmen, though they live in close quarters, do generally have a large degree of open space accessible to each of them. The minimum space argument, however, may arise out of cultural imperialism as well, a concern which must be handled with due care.