Also, about nukes in space, the explosions from any kind of bomb are naturally chaotic and are subject to having an uneven explosion. The armors of the 24th-25th century would probably be built directly around stoping such forces, also to prevent the ship from ripping apart if any compartment has a breach.
Nukes in space will have almost completely spherical detonations; the amount of energy released is so large that it won't be anything but without rather extreme measures, like including material that is heavily reflective to X-rays in part of the casing. The chaotic nature of explosions that you are used to is because a) you mostly see events which are very low energy, and b) there's this thing called air that happens to be in the way. In space with nukes, both of those things are no longer true.
Therefore indirect weapons fire would be almost completely useless, unless of course it was a cosmic force like that of a subspace rift bomb (like the Trinity Torpedo from Into the Depths of Hell). Still however, a focused blast would be required, and this would mean to down-size the blast radius tremendously using some sort of device. Also a direct impact would be required so that the target would be caught in the weapon's blast's point-of-origin.
If a shaped charge is pointed in your direction, it will affect you significantly from farther away than an unfocused weapon will. Shaped charges are all about getting more energy to where its needed, with as little waste as possible, either to limit damage to the surroundings, or to get away with using a smaller charge. To do significant damage, a focused blast is
not required. It doesn't really matter if you catch 40% of the energy or 80% of it when the 40% is more than enough to vaporize or heavily damage you.
So, yeah. That's why the explosions are smaller.
No, it's because there's no air, and a neat thing called the inverse square law. Go pick up a physics textbook and think about more detailed reasons why.
And even if the GTVA got to one of the planets or moons and bombed a city, they would have to use similar directed weapons fire, or the blasts would spatter uselessly across the frame of the buildings. Which most likely, especially if they are military, would have more armor than a capital class of similar size.
Civilian buildings are not designed to stand up to direct nuclear attack; it's worthless to do so, because of the sheer rarity of such attacks and the fantastic expense involved, even in the FS universe - remember, even with the industrial resources of entire solar systems, the GTVA does not have an especially large fleet. Most military buildings won't be armored like ships are either; again, it's almost certainly just too expensive.
