Yes, HH, I was about to say before snowdrop got in . A “1Kg is the same as 1Litre” ???
Try dropping a litre block of polystyrene on your head, then do the same with a litre block of lead!! They would be the same size but the polystyrene would weigh next to nothing. A litre of lead would weigh in at about 11 Kilograms and would cause your head some serious damage.
Just a point of interest though (or maybe not, depends on how interested you are in compost!)
The bags have got a lot smaller over the past couple of years, down from 80l to 60l , elf and safety I guess!
Or maybe I have wagon wheel syndrome:-)
Sometimes water gets in after the compost has been packed. If it did that would definitely increase the weight a lot. I suppose it must start off dry and what happens then depends on how its stored at the garden centre. I know that some compost brands are much better than others. I bought a cheap bag last week and it's hardly composted at all, full of sticks and large pieces of uncomposted material. I guess you get what you pay for.
Strangely, barbeque charcoal is sometimes sold by volume although weight would be more relevant as how much energy you get for your buck is what matters.