population density

I want to explain how many people live in one city and compare it with the other city. In doing so, what is the natural way to explain it?

Seoul has a bigger/or population density of 20000 per km2 than Tokyo with population density of 10000 km

Seouls' population density is higher than Tokyo's. That is , while the number of people who live one kilometer is 2000 , the number of people who live in Tokyo per kilometer is 10000.

Your first sentence looks pretty good to me, halmom, but I'd probably use "greater" rather than "bigger": Seoul's population density (20,000 per km2) is greater than Tokyo's population density (10,000 per km2).

I like your shorter second version if you don't need to specify the density of each city. I'd use "greater" rather than "higher": Seoul's population density is greater than Tokyo's.