Well, a nuke has a massive blast radius, period. And actually, Star Wars space combat is short ranged compared to realistic space combat. I don't think ships duke it out over several thousand kilometer ranges in Star Wars. Fights tend to be close, brutal slugging matches with ships often passing almost close enough to have their crews spit at the other ship. Look at the opening scene of Episode 3. The CIS flagship is fairly small, and half of it gets crash-landed on Coruscant. The Eclipse-class SSD is only 17km long, and it's one of the biggest, most feared ships in the Star Wars universe.Star Wars doesn't have massive distances in it, like a realistic space battle would. No, their fights are close and brutal so George Lucas can fit more ships in the camera's view for us to pay money to see.A SHIVA will definitely hurt a Star Destroyer. Badly. Add in a MAC to put another hole in it and you have one expensive ship destroyed.Does anyone not agree that a MAC will put a fairly big hole in a Star Destroyer?
A MAC would put a hole in an ISD, to be sure. But it wouldn't one hit kill it. The Empire's front line ship is far bigger then even the UNSC's largest ship. Not to mention they have shields rivaling the Covies, if not flat out surpassing them (Allthough, blocking kinetic energy seems to be a weak point for SW ships, nevertheless their shields would slow down a MAC round). Aswell an ISD has a FAR greater kill range then a UNSC ship, the only weapon the UNSC weapon that could rival an ISD in range is their MAC. Their nukes, railguns, and archers wouldn't work at such extreme ranges.
And, while in the movies the ship combat is very close, in the books it doesn't specify that often. And so one could deduce SW takes a step towards realism, seeing as how it's been specific a turbolaser has a kill range of 75 thousand kilometers. Why give it such a long range if ships are no more then a kilometer from one another? So ultimately, a SW combat ship could easily defeat a comparable UNSC ship. The tech differance is just too great, it's even greater then the covies n humans.