I know this is a very late reply, but I couldn't help myself!
Lithuania was actually once a very diverse empire that encompassed modern-day Belarus and Ukraine. Although the Lithuanians from the core lands of Aukštaitija, Dzūkija and Samogitia formed the ruling class, they made up perhaps no more than ten percent of the population, the vast majority of whom were Eastern Slavs. The Lithuanians never forced their pagan faith, culture or language on their subjects, and left the Slavic princes to rule their respective lands, much like Alexander the Great and his satraps. In fact, most of the Grand Duchy's literature was in Ruthenian, a Slavic language.
In 1386, Jogaila, the Grand Duke of Lithuania, became the king of Poland. He agreed to settle his long quarrel with his cousin, Vytautas, and let him become the Grand Duke in 1392. This began the four-hundred year-long union between the two countries. Vytautas was a great, long-reigning ruler who put Lithuania first. His lifelong dream, however was to become the king of Lithuania. He almost realized that dream when the Pope sent him a crown. Unfortunately, it was intercepted and stolen by agents of the Polish nobility, who saw it in their best interests to keep the vast Lithuanian realm weak and dependent on Poland. Vytautas died four years later in 1430.
After Vytautas' death, the kings of Poland also held the title of the Grand Duke of Lithuania. Despite being ethnic Lithuanians who spoke their ancestral language, the Polish kings of the Jageillonian dynasty were not as invested in Lithuania as they were in Poland. This contributed in part to a long process of Lithuania's decline as a great European power.
During a series of wars between 1492-1582, Lithuania suffered a string of defeats at the hands of the increasingly powerful Grand Duchy of Moscow, who, being Slavic, saw themselves as the natural rulers of all "Slavdom", and not the Baltic Lithuanians. Having lost the initiative as well as thousands of valuable troops, the Lithuanians faced a perilous situation. They became increasingly dependent on Poland, who in turn demanded concessions in the form of Ukraine. Lithuania was now left with Belarus and Lithuania proper. Together, Lithuania and Poland were able to turn the tide of the war and preserve Lithuania’s existence, but it came at a price. Lithuania’s power became eclipsed by Poland and Russia’s.
Poland and Lithuania united to become a single polity in 1569. During this time, the Polish-Lithuanian Commonwealth became the largest and one of the most powerful states in Europe, but Lithuania itself never regained the zenith it enjoyed before 1430.
The Polish-Lithuanian Golden Age lasted from 1569 until 1648, with the outbreak of the Cossack Uprising. The Commonwealth's army along with its best troops was annihilated at the Battle of Batoh and the ensuing massacre. Russia invaded Poland in 1654. Sweden saw the Commonwealth's sudden weakness as a chance to invade in 1655. This tragic period, known as the Deluge, saw the near total destruction of the Commonwealth, and the loss of 40 percent of its population - an even larger percentage than what Lithuania or Poland suffered in World War II (33 and 16 percent, respectively). The country's cities lay completely ruined and abandoned, its subjects murdered in a blaze of savagery.
From then on, Poland-Lithuania went into a terminal decline, suffering terribly again during the Northern War between Russia and Sweden from 1700-1721. The middle of the 18th century onward saw the gradual economic and cultural recovery of the Commonwealth, but the connivance of Austria, Prussia and Russia saw the final dismemberment of the ailing nation between 1772 and 1795.
Lithuania and its former possessions were absorbed by the Russian Empire, and remained so until 1918. Because ethnic Lithuanians were always confined to the territory we now know as Lithuania, they had neither the desire nor the means to recover the much larger countries of Belarus and Ukraine.
The small, newly independent Lithuanian Republic fell to the mercy of Germany, Poland, and finally the Soviet Union. Poland seized the Vilnius region in 1920, Germany eventually helped itself to the port of Klaipėda (Memel) in 1939, and the Soviet Union annexed the rest along with Vilnius in 1940. Lithuania faced another period of misery under Nazi occupation until 1944, when its once-vibrant Jewish community was annihilated, and then again by the Soviet Union for the next forty-six years until 1991. Those years were marred by oppression, attempts at Russification and economic mismanagement.
Demographic losses are an important reason for Lithuania’s small population. Not only did huge numbers of people die during the wars of the 17th, 18th and 19th centuries, Lithuania suffered perhaps the biggest loss of life during World War II - which amounted to around one million people - or one-third of the population, a staggering figure. It rose from two million in 1945 to 3.7 million in 1991. Some of that growth can be attributed to the settlement of colonists from all over the Soviet Union. These colonists started to leave after 1991.
The post-communist years were years of continuing hardship for Lithuania as it transitioned to a liberal, market economy, its streets ruled by criminal gangs during the “Explosive 1990s”. Although now it’s a “developed” nation by global standards, Lithuania still faces challenges such as emigration and a low birth rate, and its population dropped from 3.7 million people in 1991 to 2.9 million as of 2021. The European Union opened a door of opportunity for hundreds of thousands of Lithuanians struggling to make ends meet - a situation shared many countries of the former Eastern Bloc - and so they left their homeland for greener pastures.
I hope that answers your question.