??
I said (well quoted, actually): " Mars Bar's current size is . . . 51g." (Post #14)
Not sure whether you are disagreeing or not, or just adding colour. Various sizes of mini-mars bar and various multi-packs are also available, and have been for most of our lifetimes. So maybe you latched on to the OP's request (Post #13) not to get him started on 4-pack bar sizes, in which case: understood.
But (and trying to haul back to this thread's topic) the packaging of any size of Mars Bar is such that the size of the bar therein is as expected, unlike soap powder, and other granulated or liquid items sold (by volume or weight) in opaque packaging.
Now back to the much more interesting issue of volume shrinkage of fuel with temperature and the right time of day to buy.