Large ships with big weapons have a huge advantage against lots of smaller ships with smaller weapons, even with equivalent point size. This is designed to try and balance that a bit. It also leans into the trope of big guns being unable to track small fast targets, even if speed of light weapons shouldn't have this limitation.

The bigger the gun, the more difficulty it has tracking small targets

Weapons above size class 2 have difficulty targeting small targets. Targets smaller than this minimum cannot be targeted as long as they have at least thrust 4.