Trouble, the percentage that the attacker and the defender lose sum up to 100% or a bit more (roughly 105%, I think, when they are of similar strength). In extreme cases, like the one that you describe, one side loses around 99%, so the other side loses roughly 1%.
You say that the defenders lost 1/3 of what the attacker lost, so 1/3 of 500k-1M, which is roughly 200k-300k, meaning that your defenders lost under 0.5%. If observed as 200k-300k, it looks a lot, but it's really observed as a percentage, and 0.5% is really not much.
As for Fireships, mentioned above, expendable offence is the best defence against them, as they are harder to be killed by Fireships and give far less PvPs to the attacker, making your haven unattractive in terms of FS-won PvPs. I tend to have all of my Marauders out with my def, for this exact purpose.
I've upgraded my Mars to level 30, so their def strength is decent (in case a real attack hits my haven), but their main purpose remains softening the blows from FS hits and deflating the amount of PvPs that FSers get. When I get to too many Mars, I kill majority of them on a prize. They're really not good for anything else.