Hey, not an expert on this, but a lot of goes back to its historical routes.

Throughout imperial Russia’s history, European Russia began colonising the east. A lot of the east contained tribal nomands on great steppes, which Imperial Russians could colonise easily enough.

But that accounts for the large swathes of land that are largely uninhabited. The imperial Russian Empire also annexed nations along its borders. At the time imperial Russia fell it contained some 194 different ethnic groups and religions.

Lenin described Russia as a “prison of nations” as if it was forcibly keeping all these potential nations within the empire under force. So how keep all these groups together?

1. Imperial Russia’s policy towards its different people and nations was widely varied. Some groups they privileged and were given a lot of autonomy. For the most part conquered people kept their religion, (might have this wrong but of the top of my head there were 20million “russian” muslims). For people co-opted into the empire, it’s not as big a deal if you’re life doesn’t change that much.

2. Administrative weaknesses of the imperial government might have actually helped here. Imperial Russia had a underdeveloped bureaucracy, and what this meant was that they couldn’t effectively control everything every minute detail of local administration. What they did do was co-opt local leaders and their own administration. So it’s not like a place would be conquered, and all of a sudden they’re setting up russian institutions and flooding the place and get everyone to speak Russia. I don’t think they would have been capable of that. (Russification did become a policy in the 1860s I think, but I think that was directed mainly at poles and Jews).

3. Military might. By the 20th century Russia’s military was outdated and weak by European standards, but it was still huge and strong compared to smaller groups, and they were not afraid to crush any dissent violently. Poland nationalists were often being crushed, and I think the caucus region was particularity nationalistic and they’d spend years fighting them. But in the end Russia’s military would win.

So that sets up a massive Russia for the soviets to take over - and as 20th century transport and communication technology improved, a larger area becomes easier to hold together. It’s worth nothing thay under the communists it wasn’t just Russia - but a series of created nations all united - so some parts of the russian empire did nationalise and become their own country - it’s just they were their own country within the Soviet Union.

Post soviet collapse national countries split off - Eastern European is well known, but you also have a lot of the Islamic southern side of Russia going, like Turkmenistan’s, Kazakhstan. Georgia and armies and those places have gome as well.

So post soviet collapse Russia did shrink - it’s just that through their centuries of imperial colonising, there’s a lot of ethnically and nationally russian people spread to the east of Russia for it to have held together as a nation

Also don’t think of Russia as being the massive size it was forever - they slowly colonised to the east - only founded their eastern port city Vladivostok in like 1860.

Around that time I think Imperial China was in decline, so they wouldn’t have been capable of a significant invasion - and earlier

To the south of China you have the Middle East - and they were weakened by otomen and British colonialism. So the big contender to to Russia’s souther borders was the ottomen empire. But the ottomans and Russians also were geographically close within Europe - so if the ottomans tried to invade through persia - it would also Mean war on their European fronts, so it wouldn’t make sense for them to set up a front on Russia’s southern Middle Eastern borders (other than in the caucuses).

So if by the time Russia finished colonising all of its east it’s only contenders were a weak China, and the ottomen empire which would have been caught up in European politics of the day, it was pretty safe from invasion. Also don’t underestimate how good europe militarily was compared to other countries. Russia may have been a shit European power, but imperial Russia was still a great power, and woth a population around then of 120 million (I think) it’s not like smaller countries like Persia or Afghanistan have the ability to walk on and invade. It might be slow for Russia to mobilise given its size, but when it gets there it would fuck you up, so no ones going to invade