Heey that's what King George of England thought. It's actually a good description of much of the revolutionary war: "As the British retraced their 16-mile journey, their lines were constantly beset by Patriot marksmen firing at them Indian-style from behind trees, rocks, and stone walls. At Lexington, Captain Parker’s militia had its revenge, killing several British soldiers as the Red Coats hastily marched through his town. By the time the British finally reached the safety of Boston, nearly 300 British soldiers had been killed, wounded, or were missing in action. The Patriots suffered fewer than 100 casualties." "The American Revolution Begins", History.com, 19 April 2009. http://www.history.com/this-day-in-history/the-american-revo.... Another example is found in Morgan's Riflemen, who famously used guerrilla tactics to disrupt supply lines and pick off British targets. If you want a newer example, see the Vietnam War. With relatively little backing, a relatively small force of Viet Cong fought the US to a standstill.
I would postulate that this is due to a relatively simple fact: the surface area for an enemy to assault is directly grows exponentially as a function of the size. Don't believe this? Hear me out. As a guerilla force, I would be fairly independent. I can replace leaders with minimal interruption, march on my stomach rather than dealing with complicated supply lines, have little bureaucratic infrastructure, and am not bound by the rules of a nation (those of warfare and others). As a country, however, I have many more difficulties. Not only do I have the above issues, but I also have to navigate complicated foreign policy. What if the guerrillas gain the support of a foreign power (e.g. the US during Revolutionary War from France; Viet Cong from China and Russia during Vietnam War)? How to I keep from indirectly attacking one of their allies? The Korean War is an example of this: fought to a standstill with an eventual "resolution" of splitting the territory. This is for a simple reason: US-Soviet tensions meant neither side wanted to use a nuclear option, literally or metaphorically, because they feared excessively provoking the other side.

In conclusion, guerrillas are bound by none of these rules. This means that yes, a small force with weapons can defeat that of a government, even if greatly outnumbered. Our country's founders, having just taken advantage of this fact, wanted to protect it just in case a tyrant took over. Jefferson tells us that "when a government becomes destructive of these ends [securing the rights of life, liberty, and the pursuit of happiness], it is the right of the people to alter or abolish it". The second amendment prefaces the right to bear arms with the statement, "...being necessary to the security of a free State...". This is important: FREE state. A tyrant has no such need, and this is for good reason: a militia is composed of citizens, not enlisted military. This means it is under much less direct control of the government. This security is against a tyrant just as much as a foreign body. This security is to secure the "free" part of that statement, not just a nation's borders and foreign policy. This security is to secure our freedom.