Because there is forest everywhere)) I'm joking. It is really cold to live here.

First I should fix you a little: Почему население России относительно небольшой по сравнению с размером? You should write "небольшоЕ", because in Russian the word "население" has a neuter gender. "НебольшоЙ" should be used with a nouns of masculine gender, "НебольшАЯ" - with nouns of feminine. The explaination of this fact is simple: the 20 th century was one of the most horrible and bloody period of our history. At the beginning of the 20th century the population of Russian Empire was about 130 mln (in US - about 80 mln). The following events made a great negative effect on the population of Russia: -1st World War; -Civil War in 20s; -Famine of 20s-30s; -Collectivization; -Stalin's repressions; -2nd World War (the most exhausting factor). Historians say these reasons caused about 140-150 mln people losses in the population of Russia. Hope it will never happen again.