SilverWook said:

History has proven a small determined group of people can be a constant thorn in the side of a larger, more powerful foe. We never saw the entire rebel fleet until Return of the Jedi, so it's clear there was more than one base of operations.

A powerful WWII vessel could be sunk by a well timed torpedo or bomb drop, so the DS getting blown up isn't that much of a stretch.

Allowing the Death Star to track the Falcon was an attempt to lure the battlestation into a vulnerable situation, away from the rest of the Imperial fleet. (And bank on Tarkin's overconfidence.) The window of opportunity to exploit any weakness found in the plans wasn't going to be very long either way.

But even the fleet in the Return of the Jedi seems to small to me. The picture that's painted of the galaxy in the movies is one of civilization on an incredibly huge scale. It's hundreds of planets and the capital is a planet that's one big city with skyscapers that are two miles tall in some cases. Using the rebel fleet to fight against the government of that would be like five guys trying to take over Manhattan with just handguns.

Also Lei didn't know that there was a weakness, she was just hoping there could be a weakness. But the weakness in the movie is so incredible that she would have had no way of knowing that there was even the slighest chance that they could destroy the Death Star with the small forces that they had.

Also, when the Death Star is being attacked, nobody on it is saying "hey, what's going on here? What are they trying to do? They can't hurt us with such a small number of small fighters? Have they found some hidden weakness that we don't know about? What are these guys up to?"

And Grand Moff Tarken says "I think you overestimate their chances." But he doesn't say what "chances" he's talking about? Their chances of doing what? Does he know about the weakness or does he know that there might be some weakness and the rebels can blow the whole thing up?