Nuclear Weapons

All polities have nulcear weapons, and are prepared to use them in extreme circumstances. Nuclear weapons have not been much used in recent human wars because most wars have been about economics, and such weapons would be inappropriate.
The vast majority of nuclear warheads are low yield and optimised for ship to ship combat. Only capital ships carry them and Prior to the First Exterminator War there was no military requirement for larger warheads. Additional yield was thought to be of marginal benefit in space and in order to deliver a warhead to a planet an attacker have to fight the defending fleet first.
The thinking was that if the defender doesn't have a fleet then a side will win merely by by controlling orbital space. So there's no cost effective purpose to large yield nuclear weapons.

The First Exterminator War 3196-3200 changed many assumptions - largely the stress of an existential threat and the large size and power of the Exterminator Super Battleship meant that nuclear weapons were not only used on an unprecedented scale against the Exterminator menace but there was a need for Very High Yield Weapons. It took some time to ramp up production of VHY warheads but when they were used they had a considerable effect - many historians consider them to be one of the decisive weapons of the War.