You're never going to have a million tons of armor. The construction rules limit you ships no bigger than 2,5 million tons and amounts of armor that look basically like a tinfoil covering on top of everything else, if not an outright paint job. (The absolute most you can carry on a WarShip, assuming its internal structure is maxed out, is its maximum thrust x 0.06% of its total mass.) No, I think that's the whole ship...and yes, it has some decidedly balloon-ish aspects.
I am aware of that - I just wanted to show how the ship is a big too big for its mass.
GURPS Traveller starship construction rules - the only ones I have that take both volume and mass into account - tend to make most ships mass about 0.25 to 0.5 tons per cubic meter (3.5 to 7 tons per 14 m3 "displacement ton"); I think that´s a pretty good ballpark estimate, which also happens to overlap with David Weber´s overall density for warships of 0.25 for his Honorverse novels.
Using the 0.25 figure, a 1 million ton warship would have 4 million cubic meters - say 500 meters long, 100 wide and 80 high. That´s 196,000 square meters of surface area.
With a max thrust of 5, that ship would have up to 3,000 tons of armor, which comes to, let´s say, 300 cubic meters - off the top of my head I´m arriving at something like 1.3 or 1.4 millimeters of armor for these figures.
And even with a density of 2, with every dimension half as long, you´d end up with 5-6 millimeters of armor.
And all that assumes that armor is only about 1.25 times as dense as iron.
I think this is the right moment for one these: :o
