It's not difficult, in the pre World War I period, Russia had a higher total fertility rate that was the highest in Europe, and was 70% higher than that found in the average in the Anglosphere settler societies (Australia, Canada, New Zealand, United States). In many ways, Russia's demographic trends were 30-40 years behind Germany and Scandinavia, as it still had a higher infant mortality rate and death rate. Despite this, the natural growth of the population was 1.8% per annum, a rate higher than any of its European peers. This allowed the population of the Russian Empire to grow by 50% in the 25 years during the reign of Nicholas II. Emigration increased, but over half of these were Jews, with the rest being Poles, Finns, Lithuanians and ethnic Germans. Ethnic Russians and Ukrainians accounted for less than 5% of all emigrants from the Russian Empire during this period.
The easiest way to grow the population of Russia is to butterfly away the world wars and the Soviet Union. If the empire follows the same demographic trends as say the Balkans did (excepting the wars), its population would have numbered around 450 million today. If the country undergoes a rapid demographic transition in the 1960s, the population would still be 355 million today. If the first figure is taken into account, at least 200 million of these would be ethnic-Russians and another 80 million would be Ukrainian.
Keep in mind that in World War I, Russia lost between 2.8 to 3.4 million people, couple that with the Russian Civil War and Pre-Stalinist Purges and famines adding another 9 million. Stalin was responsible for the death of another 20 million and World War II another 27 million were killed. That alone gives you 59 million unnecessary deaths.