The concept that's being breached here is called "parallelism." It's a part of grammar, not just idiom, so I think that the author's sentence may actually be grammatically incorrect. Here's another example of parallelism in action:
Today, we went biking, surfing, and kayaking.
Notice that each verb is a single word with an -ing suffix.
Today, we went out to bike, then surfed, and went kayaking.
This isn't as awkward as the sentence about Russia, but it's still incorrect because the three verb phrases are inconsistent.
Russia is the northernmost large and populous country in the world; much of it is cold and/or dry.
Parallelism is broken here because "northernmost" is a superlative while "large" and "populous" are both regular adjectives. If he would have said "northernmost, largest and most populous" it would be in parallel, although factually incorrect.