The last Chinese dynasty, the Qing Dynasty (1644-1912), was ruled by a minority called the Manchu people. The were from northeast China, and limited population settlement there. Compared to the region south of Beijing, it was sparsely populated for this reason.
In the late 1800s, Russia started to develop Harbin and other Cities in Northeast China as a colony. Economic problems in the non-colonial Chinese regions led to population growth in many colonial outposts, most notably Hong Kong and Shanghai, but also Qingdao, Shandong, Xiamen, Tianjin, and the Russian Cities in the Northeast. The fall of the Qing in 1912 ended regulations preventing migration to the Northeast, and accelerated economic decline, coupled with population growth, led to increased migration into foreign controlled zones.
In the 1930s, Japan came to control Manchuria, and proceeded with economic development. By the time the communists founded the PRC in 1949, the Northeast had been under industrial development from first the Russians, then the Japanese. In 1949, it was the most industrially advanced region in China.
When the PRC embraced the Soviet economic model, and received Soviet economic assistance and advisors, much of the industrial development also occurred in the northeast as it was close to the USSR, and it was already the most industrial part of the country. Many professionals and unskilled laborers from other parts of China moved to the Northeast for opportunities. This spurred enormous population growth in this part of China.
Then, in the 1960s and 1970s, the CCP encouraged a high birth rate and the population of China just about doubled to a billion people. So the Northeast has a large population for two reasons 1) China has a ton of people in general and 2) there was (is) a lot of economic opportunity there.
Let's compare this with Russia. Most economic development occurred near Moscow and Western Europe. Siberia was viewed as a place for prisoners and natural resource extraction. And Russia doesn't have the huge number of people that China has.
Northeast China has more people because it is in China, and China has more people than Russia. Also, the Northeast had a lot of economic opportunities in the first decades of the PRC (1950-1990) because it was the location of economic development due to its location near Russia and the Japanese empire.