I support Serenitis opinion.
I too am facing very strong expeditions on a regular basis from multiple factions, they go from evenly matched to superior in term of fleet strength comparison. Doctrines are setup for max fleet size and the best (and biggest) ships. Just "Strong" expeditions are always outmatched by orbital defences.
Colonies are size 8 to 10, they all receive ships from a colony with a pristine nanoforge+orbital work, fleet logistics is maxed out for the one in charge of the colony. (Either me or an alpha core)
They have a level 3 space station and patrol HQ /Military base/High Command (Since military bases don't increase fleet size according to the ingame info, I have been experimenting to potentially optimize income)
Their raiding/boarding strength is usually outmatched, which means that I at most lose my orbital station, (which is -3 stability for 90 days, that equals to about 100k-200k/month less for 3 months)
I get about 1 or 2 raids every 2 months. Usually targetting the same colony despite there being 9 targets. (All with free port and high market share of various products, and all size 8 plus)
Even if I did have an industry that increased local fleet size to levels that would allow my colonies to easily beat the repeated expeditions, that would just create a new problem. Expeditions would be almost completely and utterly pointless and add no fun to the gameplay experience. (Currently, they subtract from the gameplay experience, so it would still be a step forward)