It's mostly a geographical thing, Siberia (most of Russia's land) is largely uninhabited barring a few native tribes who were overwhelmingly outmatched in terms of manpower, training and equipment by Russia.

Other Empires, notably the Mongols, had potential to take Siberia themselves but were busy picking fights in places where people actually existed, in large amounts.

Other states had potential but were busy with their own politics, e.g. China was not expansionist as it was content remaining isolated from the world, plus most other suitable candidates were nowhere near suited to a harsh Siberian winter and had no idea what to go looking for that was worth the effort.

Taking land is basically VERY easy when there's no one to be taking it from.

Colonial Empires had potential to acquire more land but the logistics didn't always agree because Europe was a place where people hated each other immensely. Great Britain had frequent conflict with France and Austria, Spain had conflicts with the Moors (Pre-Reconquista actual states, Post-Reconquista Barbary Pirates and attempts to christianise Africa), Ottomans, France and the Dutch; while Portugal was...not exactly strong. Compare this to Russia who never really had overwhelmingly strong neighbours by the time it was colonising Siberia, the Mongol remnants were fractured and divided, while the other Russian principalities were tiny; the Polish-Lithuanian Commonwealth had other concerns with the Ottomans while Sweden was busy trying to win independence and remain independent from Denmark's overlordship.

An important concluding point: An Empire is not always big and strong, it's just a title adopted by the State. For example, Byzantium would still be an Empire in 1452 despite being virtually annihilated by the Ottomans by then, similarly Portugal was technically a Colonial Empire but was vastly inferior to most European powers.