The cities of Oldtown and Spoonville are the same in area and size of population.
Conclusion
Health problems caused by crowded living conditions must be as widespread in Spoonville.
Evidence (since)
Health problems caused by crowded living conditions are widespread in Oldtown.
And Oldtown and Spoonville are the same in area and in terms of population.
Evaluate
This is very explicitly an argument by comparison. The author thinks that .
Whenever we're trying to weaken or criticize a comparison, our first thought is to think of some important difference between the two things being compared.
Since the cities have the same area and population, they have the same "population density". If they each have 10,000 people over an area of 50 square miles, then they each have a population density of 200 people per square mile.
The author is looking at health problem caused by crowded living conditions, and thinking that since these cities have equal population density, they would have similar issues with crowded living conditions.
But the two cities might differ greatly in terms of how many, if any, crowded living facilities there are.
It could be that Oldtown's city area is mostly covered by uninhabited hills and forests, but there's one major city where most of its 20,000 people live. Meanwhile, in Spoonville, it's a bunch of sparse suburban and exurban neighborhoods. It's all single-family homes, so there aren't really any crowded living facilities.
Even if they both have crowded living conditions, the two cities might differ in terms of how well such facilities are run, in terms of sanitation and good public health practices.
Goal
Let's look for an answer complaining that the author is making far too confident a comparison, merely on the grounds that both cities have the same population density. They are failing to consider potential differences between the cities that would lead to very different levels of concern in regards to health problems caused by crowded living conditions.