There is a scene in ANH when Luke fires his weapons at the surface of the Death Star. A small explosion results, Luke's X-Wing flies through the cloud, and Luke then reports having gotten "a little cooked".
As you can see below, these were not huge explosions, despite the claims by some that they were Hiroshima-scale. Note the X-Wing's bolt striking the surface in the shot below (and don't forget the range of Star Wars weapons, giving an upper limit on just how far the shots could've gone):
Those explosions are not even close to the size of the blast of an atomic bomb. Note also what appears to be a gun turret off to the right in that second picture, and not far from the explosions. That is not a huge kilometer-tall tower . . . the top of the turrets we saw firing on X-Wings ought to be perhaps one or two decks tall, and most likely just one.
(ANH-dsturret.avi, 360kb, DivX 5.0.3)
Some pro-Wars debaters attempt to claim that the shots from Luke's X-Wing must have vaporized hull material, since armor is not flammable. Therefore, they say, the explosion and resulting expanding gases must mean that about a cubic meter's worth of hull material was vaporized.
However, nowhere is it stated that Luke vaporized hull material, and to claim that secondary explosions are impossible is absolutely crazy. Nowhere else in the entire canon do we see explosions of this magnitude from fighter weapons impacting against hulls, not even other fighter hulls, which by the logic used in this example should be instantly and rapidly vaporized by the slightest touch. Obviously, Luke's shots caused an explosion, but an explosion of what? We aren't sure what he was shooting at . . . he could have hit the Imperial equivalent of a fuel storage pod, a turbolaser cannon, or any number of things that would be likely to cause a secondary explosion. The sheer scale of even the smallish explosion above should've clued them in . . . after all, most of the shots from fighters in ANH didn't produce anything even close to that level of destruction.
Naturally, with such foolish assumptions, they come up with foolish numbers, claiming 600 GW for a single X-Wing cannon, and claiming that X-Wings are therefore more powerful in *seconds* than the entire United States. I guess the X-Wings are hiding their giant fusion reactors and enormous fuel stores necessary for such a thing.
Though Warsies continually attempt to deny the above concept, it is nevertheless
supported by the canon novel. ANH, Chapter XII:
Bolts flew from the tiny vessel's weapons. One started a huge fire on the dim surface below, which would burn until the crew of the station could shut off the flow of air to the damaged section.
Luke's glee turned to terror as he realized he couldn't swerve his craft in time to avoid passing through the fireball of unknown composition. "Pull out, Luke, pull out!" Biggs was screaming at him. But despite commands to shift course, the automatic pressors wouldn't allow the necessary centrifugal force. His fighter plunged into the expanding balls of superheated gases.
Then he was through and clear, on the other side. A rapid check of his controls enabled him to relax. Passage through the intense heat had not been insufficient to damage anything vital, though all four wings bore streaks of black, carbonized testimony to the nearness of his escape.
Hell-flowers bloomed outside his ship as he swung it up and around in a sharp curve. "You all right, Luke?" came Biggs's concerned query.
"I got a little toasted, but I'm okay."
The novel also refers to the targets of opportunity being aimed for and
the explosions of those targets, contrary to the claims of some that the
entire battlestation was covered in impregnable armor, and that the
fighters were simply shooting at armor for the sheer pleasure of it.
Indeed, simple fighter-scale weapons were blowing holes through the armor and
producing "subsidiary" (i.e. "secondary) explosions,
as per the second paragraph of the following:
Energy bolts and sun-bright beams continued to create a chromatic maze in the space above the station as the rebel fighters crisscrossed back and forth over its surface, firing at whatever looked like a decent target. Two of the tiny craft concentrated on a power terminal. It blew up, throwing lightning-sized electric arcs from the station's innards.
Inside, troopers, mechanicals, and equipment were blown in all directions by subsidiary explosions as the effects of the blast traveled back down various conduits and cables. Where the explosion had hulled the station, escaping atmosphere sucked helpless soldiers and 'droids out into a bottomless black tomb.
Another commonly attempted counter-argument is to claim that, in AoTC, Slave I demonstrated far more than 60 gigajoule blaster weaponry. In fact, this is not correct. Using Wong's own asteroid calculator (since I explicitly haven't worked that scene yet), and inputting the overestimate of 10 meters for the asteroid we see to be destroyed (just eyeballing the fighter as 4 meters wide), one finds that the required cratering energy (i.e. the energy required to make a crater equal to the radius, which is pretty much the effect seen) would be only .2 tons (0.84 gigajoules, or 840 megajoules) for solid granite. Even if we assume that these are magic planetary rings made of nickel-iron (as opposed to common rings made of far lesser materials, usually ice or sometimes collections of dust and dirt clods), we're only up to 19.7 gigajoules for the overestimate asteroid, from a vehicle significantly larger than a fighter.
Some humor from the non-canon:
The Episode II Incredible Cross Sections book throws another absurdity onto the fighter firepower flames, claiming one kiloton fighter weapons. That, boys and girls, is approximately 1/15th of Hiroshima! Funny, I never saw Hiroshima level events in any of the space battles, and I certainly didn't see anything even remotely like Hiroshima on Hoth, Endor, or Geonosis.
Of course, this is the same ICS that claims 200 gigaton weapons for the prequel-era combat vessels such as the proto-ISD Acclamator, without realizing the inherent absurdity. See, some claim that Star Wars fighters can't actually do anything of value to a Star Wars capital ship unless the shields are down, and that the shields can't be brought down by fighters.
However, such claims are patently absurd. There is no point to fighters, unless they can be employed against the ships before ship-to-ship combat ensues . . . especially if there's such a wide gulf between fighter firepower and ship firepower.
And yet, we've seen fighters employed against starships with great success, sometimes even by accident:
1. In TPM, the droid control ship of Mr. "Nothing can get through our shield" (spoken just before the end of combat) lost a huge dish that exploded when hit by Naboo fighter weapons at the start of the battle, and his ship sufferred damage from a collision by a retarded droid fighter midway through:
2. Rebel fighter weaponry (and a fighter collision) was more than successful in damaging the unshielded Executor bridge region.
Though the above scenes all involve either fighter torpedoes or direct fighter impact, fighters do indeed use their beam weapons against ships, as well . . . though we haven't seen them have quite so visible an effect. In TPM, the Naboo fighters flung many a bolt at the droid control ship. Of course, even if you just count their torpedoes, the droid control ship could not have been all that tough in comparison to fighters . . . as seen by the result of Anakin's two-torpedo shot (in atmosphere, no less, where blast effects come into play), Naboo fighter torpedoes are decidedly sub-kiloton. And yet, even the weaker cannons were capable of blowing apart droid fighters!
Further, if ships are vulnerable to fighter weapons, then it is unlikely that shipboard weapons are going to be dozens of orders of magnitude more powerful, as is claimed now. For instance, the E2ICS claims of 200 gigatons for Acclamator weapons work out to 836,800,000,000 gigajoules . . . almost 14 billion times stronger than the high-end Warsie-made fighter firepower figures mentioned above. The ICS does bump up that firepower number a bit, so that starship weapons are made a mere 200,000,000 times stronger . . . and yet, we're supposed to accept that in light of starship damage by fighters???
Special thanks to "Razor" for the discussions regarding to the non-canon points.