From Starfleet Commander
The Large Decoy is a special defense. Essentially a larger and more attack-absorptive version of the Decoy, it has no attack, only shield and hull. Like the Decoy, you may only have one Large Decoy per planet.
Its primary purpose is the same as that of the Decoy--to absorb hits from opposing attackers so your other defenses and ships do not. It can absorb larger hits than the Decoy can without taking hull damage, and even when damage gets through, its massive hull helps keep it alive.
The Large Decoy is relatively expensive and has no attack value. It does not draw fire from opposing fleets, it is merely hit randomly like any other defense. Therefore if you have a lot of units, the odds of any attacks hitting the Large Decoy become small.
This unit can be very amusing for a defender when the attacking force has a total fire power less than the shield strength of the Large Decoy. The defender will win with not a shot fired.