Colonialism is a large factor. Britain and other countries colonised massive areas of land. Russia did the same but thet focused on east and into Siberia, which was valuable to the Empire for its fur trade.

The focus east was also because west would be facing powerful european countries, no country like that was east except the Golden horde and China, making it less competative.

But why is it still so large today? Well Russia differed to other colony stratagies, by using integration and assimilation of the indiginious, causing a more unified national identity.