Say Russia for some reason wanted to be bigger, and then be more secure by having an even larger population. Through a higher birthrate and more net immigration a country can be growing in population without having to conquer other places.
This isn't like the 1700's and 1800's where there wasn't enough farm output to feed more people and all the farmland was already being used in Europe. So to expand countries had to conquer some other country.
If you look since the year 1900, US farm yields per acre have risen 12 times. That is why even as the US population is growing our farmland is declining because each year there is an ~3% increase in yields per acre, but our population usually grows ~1%. Of course the USA also exports food so we don't necessarily return all these fields to wild nature. But this is why we are returning the buffalo to the prairies is we no longer need all the farmland and we need less every year.
Russia is also a huge food producer, main exporter. As they put in place Western farming science and private farm ownership after the Cold War, Russian farm output has been rocketing up. Yet their population of Russia is flat since 1990. Turning them from a food importer into a huge food exporter.
The US is expanding our empire by a few countries a decade. But when we try to do it by conquering other places by force of arms it is insanely costly. And as seen in Afghanistan not a sure thing that we will hold it.
