The way fleets scale varies depending on the kind of fleet.
The most video gamey scaling are the bounties which, IIRC, scale minorly off time and majorly off player level and how many bounties the player has taken down.
Pirate and pather patrols on the fringe, as well as pirate raids, are stronger based on the size of their originating space station which grows the longer it stays operational.
Meanwhile, the strength of patrols in core systems from major factions, permanent pirate colonies, and your own patrols are based on their colonies. Size is based on patrol HQ(weak), military base(average), and High command(strong) on a planet. Quality is based on Heavy industry/Orbital bases and the best nanoforge owned by the faction. Size, ship quality, and officer numbers/level are also modified by faction doctrines which all go into faction system presence.
Spoiler
Lastly, The remnants strength in a system is based on the station in the system. Fully operational (red beacon) systems are the strongest, damaged stations (orange beacon) are medium strength, and no station (yellow beacon) are the weakest.