I had trouble trying to get zillions of pirate ships to appear at the pirate base... even after both the computer and I fed them over 100,000 credits each...
What I am preety sure was happening, was that in your game... the pirates would raid planets... and then what was left of the raiding party returned to the pirate base. In my game the pirates never got the chance to return to the pirate base... (they allways attacked me... was playing against the new 5x resources ai.... ai always could out bounty me) so the pirate defence fleet in my game never grew, while in your game the pirate fleet kept growing for every sucessfull raid.
In other words... the pirate defence fleet only grows if ships from a raid make it back to the base.