The main reason for Russian expansion in 1800s was the end and attempt to gain a warm-water port.
By the 1800s, Russia was already one the largest Empires in the world. It had large swathes of land in Europe and Asia and had tremendous natural resources in terms of timber, fresh water, minerals etc.
However, despite it's immense size Russia lacked a warm-water port. Both it's ports in the Pacific and the north Atlantic were closed every year due to ice.
This several restricted Russia's trading prospects and posed a natural security threat in terms of a blockade.
Within a few years, Russia was able to acquire a port in the Black Sea in Crimea.