The vastness of Russia, although perhaps an advantage if managed well, did not make matters easy for Nicholas II. His grandfather, Alexander II, was largely responsible for the implementation of the mostly unpopular policy of Russification, forcing non-indigenous citizens of the Empire to speak Russian, dress Russian, even adopt Russian customs. This policy was particularly disliked in areas such as Poland, Finland and Latvia. Interestingly, of the Russian Empire, only half of the population was actually originally Russian, or of Moscovy descent. This was a potential time bomb, as seen in the 19th century in Europe during the rise if nationalism.
The lack of a significant bourgeoisie meant the intellectuals and wealthy who were more likely to be placated by promises of increased democracy, such as the mostly ineffectual Duma, were not large enough to have any real influence over a proletariat who by 1916 clearly felt that enough was enough. After 1905 when Tsar Nicholas II made concessions which became known the October Manifesto, the middle class were moderately satisfied, and began to back him again. This most likely caused Nicholas II to forget how precarious his position was, particularly since he began to overrule the new parliament known as the Duma, and drastically cut its power so that once again the country was basically run by an autocracy. Any decisions made at the Duma had to be approved by the Tsar. This of course did not please the bourgeoisie, however, one must remember that the middle class does not particularly care for bloody scenes or lawlessness. It would appear that they took a figurative 'step back' in political affairs.
The middle class almost always originates from poverty. Thus one could conclude that a small middle class meant a large peasant/working class population. Although I do not believe that peasants are ever solely responsible for the overthrowing of an ancien regime or a revolution, I do believe they played a significant part in the revolutions in Russia. This is because of the rapid industrialisation, meaning before mainly isolated and uneducated people were heading towards cities where they were exposed to political parties such as the Socialist Revolutionary Party, who were viciously opposed to the monarchy and what they felt were the farcical concessions made previously. Another factor, probably more important, was that the wages of the working class had not risen since 1905, and urban workers were unable to provide for themselves and their families.
Finally, the decisions Nicholas II made during the First World War in an already politically unstable situation meant a total loss of confidence in him and his regime. An unfortunate, though perhaps gallant decision to lead his troops on the Front was marred by his tight grip on government affairs, and his decision to place the infamous monk known as Rasputin in charge of the country with his wife the Tsarina. These two effectively drove the country to its knees through their inability to appoint and retain competent ministers and officials. Newspapers and cartoonists of the day also insinuated an extramarital affair between them, heightening the sense of dissatisfaction already rife among the people of Russia.
By this stage, any move the Tsar could have would have been the wrong decision, and the loss at the Front with him in charge caused people to hold him personally responsible, be it fair or not. His decision to order his soldiers to fire at protesters in Petrograd was a point of no return, now he was clearly an enemy the people could unite against, and this they did.
The downfall of the Tsar was inevitable, I believe, as soon as he made it clear that he was not interested in any form of government other than an autocracy. However, many factors contributed towards the actual onset of the 'overthrow', one of the main ones being the First World War and the way he handled it. Ultimately, downfall was inevitable in any ancien regime that did not move forward in the 'natural progression' (according to Karl Marx) of things, as was seen in France, and later on, Russia.
