The Galactic Republic and then Empire is huge, containing (I think) hundreds of thousands of star systems which would each have millions to billions of sentient inhabitants.
Despite this mind boggling size, the Rebel Alliance, particularly in ANH and ESB appears to consist of at most a few thousand members only a few hundred of which are frontline fighters. Consider the handful of fighters sent against the death star (in ANH) and the paper thin line of ground troops defending a small base on Hoth which housed the entire alliance.
Does this mean the Alliance is unrepresentative of the general population of the Empire? Scaled down to current Earth sized countries, this is like a group of 2-3 people trying to overthrow the Government through guerilla attacks, bombs and assassinations. We'd call these people terrorists (and probably nut-jobs as well).
Why is the Alliance so small, and are they really the good guys or just the disgruntled elite of the Old Republic, out for some revenge?