They tried.

In 1700s Balkans was most of the time ruled by Ottoman Empire, which was active opponent of Russian Empire. Also in 18th century Russia was fighting Prussia and Sweden.

They tried to establish colonies on their own (think Alaska) but the main problem was that Russian Navy was inadequate to sufficiently support and protect colonies overseas. Because if they will need to fight English, Spain or French Navy to protect their colonies they will have no chance. They gained territories in Middle Asia and managed to capture some lands in Eastern Europe and in 19th century annexed parts of China and Korea.

Well to partially answer your question the time was ripe for a takeover in the case of the Soviet Union. Stalin had implemented massive reforms in the Soviet Union with his five year plans and multiplied the power of the country. Also with the second World War the massive Soviet manpower that had been built up had pushed it's way through to Germany and had a strong hold on Eastern Europe. Also after being attacked in both world wars Stalin did not want a repeat of history at least on his territory so he sought to create a buffer zone between him and the west. In short Russia was very able to make moves at the time and had every reason to. Lucky for you I just happened to be looking into the cold war for my APUSH exam coming up and this is partially discussed in the new textbook "Give Me Liberty". Anything not in the book I learned through discussions with my teacher who wishes we could explore the more interesting parts of history in much greater depth. As for Russia in the 1700s I would say a lack of a large attempt to grab land would be due to less optimal circumstances and a lack of an aggressive leader. However I would wait for a redditor with a better knowledge of Russia in the 1700s to give a more detailed answer.