Erm, I've never found the pirate base to be too tough, but I don't know if the number of ships there is constant or changes with the size of the raids.
Mind you, I don't use 'spam' fleets of a single ship type. 50 heavy cruisers is 500-600 fleet capacity. A mixed fleet worth the same amount of fleet capacity (with a Capital ship or two, and a mix of frigates and cruisers, including support types) will generally be a lot more effective and durable.
Oh, and you're right - you shouldn't need 20 heavy cruisers to fight off pirates every 10 minutes. For a start, they don't attack that often (

), and unless you deliberately want to be raided to level up a Cap ship, you should be able to avoid the raids most of the time by playing with the bounties on your opponents.
Even if you do get attacked every raid, your static defences should be handling most of it, with maybe a small fleet to back them up.
With all that said, there is a valid design reason for the pirate base to be fairly well-defended - if it was too easy to destroy, you might as well take pirates out of the game altogether, which I think would be a shame. They're an interesting mechanic.