More can be written about this. I found these previous discussions:

"How did the unification of the Slavic peoples under Moscovy/Russia come about?" by /u/the_gnarts about the Gathering of the Lands of Rus.

"How was Russia able to form into such a large country?" by /u/ayyum and /u/facepoundr about general factors and some about Cossacks.

Out of the FAQ, specifically about Siberia: "How did the Russians conquer Siberia? How long did it take them?" by treebalamb and facepoundr .

EDIT for future reference: "How did Russia permanently subjugate the steppes, a feat that was unachievable by multiple Chinese dynasties?" by /u/Kochevnik81 has something of a different approach to this.

This is not to discourage discussion. Further questions, data, and debate are always welcome.