The one huge flaw in your logic is that Mojang only confirmed the spatial dimensions of a block, they never once said that any minecraft material had a weight (and by extension had the same weight as in RL).
Not to mention we don't know if Minecraft blocks are solid. What if cubes are actually hollow? That would explain why you only need 9 ingots to craft a cubic meter, or why you can so easily shrink it down into a fist sized cube when you break it.
We don't know enough about Minecraft's reality to make any assumptions.