The Imperial Class Star Destroyer was developed late in the Clone Wars. It was a natural progression. It offers better protection against starfighters than the VSD. The Super Star Destroyer is a command ship. Of course it is going to be large with a navy this complex. Lets see, the head of state was helping the rebel alliance, his daughter is a rebel, General Rieekan also was a rebel. This means he has the governing body and military support for their traitorous nature. Alderaan had claimed neutrality and non aggression. When they started transmitting stolen plans and tried to smuggle a Jedi back to their home planet, this is an act of aggression. Alderaan thought they could out smart the Glorious Emperor and paid the price for their treachery. They served as a fine example for worlds that betray the Empire.