I have not encountered a single full-sized pirate encounter army in the past year, since I pretty much always maintain full-sized armies of 20 units.


In that entire time, across dozens of campaigns, every single naval pirate encounter has been a half-sized army that poses no threat at all to a player's full-sized army.


The one single time I start a new campaign with Eltharion, decide to keep control of  Yvresse, and am forced to maintain two smaller armies due to budgetary constraints, the very first naval encounter I engage with produced a full-sized pirate army.


If the size of the pirate armies is supposed to be random, this is statistically impossible, so I can only surmise it's a bug that somehow causes every single pirate encounter fought with a full-sized 20-unit army to always be half-sized.