Make a Gunboat class ship with an extremely powerful torpedo that does around the 500,000 damage but IGNORES shields.
Make that gunboat slow and fat - thus easy to kill even by a meager amount of defenders.
Make the torpedos slow. REALLY slow. (Slow as in 30-40 m/s), so defenders can flak/CD/Mine/Ram/<enteraction> them.
So people who can't be bothered to come and defend their base would lose it, but those who actually bother and give time in the defense would be able to fend off the attackers, even if they are in a smaller amount.