Density is a subtle thing, and population density is not a pure measure. Imagine I had a friend who wanted to move to Calgary but wanted to live in a new home in a low density location. I could tell them that I'd found them a place in a dissemination area with a density of 827 people per sq km, about half of the citywide average of 1592 (which includes not only road right of ways but also parks, the airport, and so on). Most new suburbs are in the 3000-4000 range, four times as dense as the new place I was able to find them! I reckon they'd be surprised when their moving truck pulled up to Telus Sky, even though everything I've said was true. Telus Sky is in a very low population density area (because the space is taken up with office towers instead).
Or to turn it around a little bit, here are three dissemination areas. Two have roughly (2016 Census) the same density, and one is a real outlier.
It seems obvious that area C is the outlier. If you look at dwelling unit density, you'd correctly reach this obvious conclusion; area A and B are both about 24 units per hectare, and area C is three times higher, 81 units per hectare.
But: we've been talking population density here. So this is wrong. Area B is the outlier; it has half the density of areas A and C -- 66 people per hectare versus 109 and 118.
Area A is in Taradale; it has 4.59 people per household. Area B is in Tuscany and has 2.91 people per household, and area C off Edmonton Trail has 1.56 people per household.
Going to the example of the Vancouver area and Marda Loop, the two census tracts you picked:
Marda Loop has 81.5% of the population density as the Vancouver area (I'll call it South Hill to give it a shorter name -- it's sort of between a couple of neighbourhoods).
But on a dwelling unit density basis, Marda Loop has 120% of the density of South Hill; 2674 vs 2230 units/sq km. On a bedroom basis, they are almost exactly equally dense -- 6489 bedrooms per sq km in Marda Loop, 6516 in South Hill (99.6% of Marda Loop.)
Going from this, the Marda Loop area is built out at fewer bedrooms per unit -- 2.42 vs. 2.92. It has much smaller household sizes; 1.95 people per household vs. 2.81 in South Hill. Which is what typically happens in densifying areas; 51% of the housing in Marda Loop was built since 2000 and only 27% in South Hill. Both areas have the same share of SFD housing - 23%, but Marda Loop has 47% apartments while 51% of the units in South Hill are what StatsCan calls "apartments in a duplex" which isn't what anyone I know thinks of as a duplex but is actually an up/down living situation where one unit is above the other, like a house that has been subdivided into an upper floor apartment or that has had a basement unit built.
The flip side is that the units in South Hill are more intensely occupied; 0.89 people per bedroom vs. 0.73 per bedroom in Marda Loop. There are a number of reasons; some could be cultural. The South Hill area is just north of the Punjabi Market area and has a substantial South Asian population -- the same trend that makes the far NE in Calgary the densest area by population density outside the core. Marda Loop has about 88% the population you would expect for an area in Calgary given the mix of unit sizes; South Hill has about 104% the population you would expect in Vancouver. But I don't think anyone would say that Taradale was built much better than Tuscany; it's just more intensely occupied.
Zooming out for a second, one of the reasons that Vancouver has higher population density is that they have more people per unit.
But it's worth noting that the gap between the cities is much smaller for homeowners than for renters; that suggests that part of this is that people are living in smaller units because that's all they can afford. The guy behind CensusMapper has a
good blog post talking about the 'suppressed households'; households that would exist if affordable housing was available (Montreal is the comparison market) but that don't exist because housing is unaffordable. Here's how 25 to 34 year olds live in the two cities:
Density is good -- both for the planet and for quality of life -- when it means that there are shops and restaurants in walking distance, or when it means that frequent transit will be supported with high ridership. But what about when density means living as an adult with your parents or a jerk roommate because you can't afford to move out? Still good for the planet, I guess. But not for your quality of life.