How does it work?

Internally, the mod checks each planet's maximum asteroid size limit, and compares each asteroid entry in the planetary asteroid spawn table, and if the given asteroid is above the limit, then it reduces the size of the asteroid to the limit itself (thus preserving the total asteroid spawn rate).

If Safer Interplanetary Routes are enabled, then the mod checks which planets were modified from the vanilla values, and then changes the routes between those planets to the interplanetary maximum asteroid size limit.

If either the source or destination for a route has asteroids more dangerous than the route itself has, then it will "fill in the blanks" with a linearly scaling value, from 0 to the level of the more dangerous planet. This is a core "space-connection" behavior, and cannot be modified.

Compatibility with Asteroid Frequency Multiplier

Safer Asteroids runs after Asteroid Frequency Multiplier, so the changes to harmless and dangerous asteroid spawn rates will apply first, then (if necessary) the dangerous asteroid sizes will be modified.

For example, both harmless and dangerous asteroids can be increased on any planet by Asteroid Frequency Multiplier, then the dangerous asteroids can be rendered harmless by Safer Asteroids; as a result, both of the additional spawn rates would then apply to the harmless asteroid chunks.

If dangerous asteroid rates are reduced before being rendered harmless, without an equivalent increase to the harmless asteroid rates, then the planet will have fewer asteroids in general (when compared to vanilla).