Ah, I have made an elementary mistake here. Being staunchly metric I do not know what an ounce is* and thought it to be a unit of volume. Google tells me it is a unit of weight.
Well, of course two things that weight the same, weigh the same and gold is denser than lead so, yes, it would be smaller too.
So, my mistake, boy is my face red.
*And will promptly forget it after this, because I just refuse to learn some arcane thing where distance is measured in un-understandable things like furlongs and body parts like feet when there is a perfectly good metric system out there where all things are decimal and you have none of that sillyness where an ounce is 1/6th of a pound but a pound is 1/14 of a stone.
But this has nothing to do with anything really.