Corrected entry: As powerful as the Enterprise is, a 229 meter vessel would not really stand a chance against a several mile long planet destroyer. Despite this, the Enterprise survives several hits with only minor damage.
Corrected entry: Spock says that they will be unable to close with the Machine because Enterprise's energy cells will draw its attention, but then later he says it ignores ships such as themselves beyond a certain size radius - which is it?
Correction: Size matters not... sorry, wrong space opera but the correction still stands. There's no way to make this claim without knowing the specific effects from the attack. The size of the ships is irrelevant. Birds brought down a jet in Manhattan not too long ago.
JC Fernandez
While you have a valid point, consider that the Constellation was turned into essentially drifting junk, after being attacked with the Planet Killer's beam. The Enterprise was attacked by the same power levels, but only gets minor internal damage. If the beam was powerful enough to slice up a planet, and lay critical waste to a starship, then the Enterprise should have met the same fate.
Movie Nut