...And some places have "lighter" snow than others.
I have seen experiments, where you fill a large glass with snow from the alps - and another with snow from the Rockies (US/Canada). When melted, there is only half the water in the glass from the Rockies - which is why it is powder-heaven for snow-freaks (such as myself).
I have no idea about the science - why it is this way.
Try that same experiment with a snow bank in late May from both places and you'll likely find it's equally compact once the snow pack is at 32F throughout and starting to form what is known in the trade as firn. [1]
Compared to rocks, ice is not that dense. You can try yourself: it's easy to pickup a big chunk of ice, even with a shovel, while a similarly sized rock is impossible to lift.
"Other substances that expand on freezing are acetic acid, silicon, gallium,[35] germanium, bismuth, plutonium and also chemical compounds that form spacious crystal lattices with tetrahedral coordination."[0]
There's more than water. A lot of things that freeze into a crystal lattice can be less dense than their liquid states. Silicon dioxide is another IIRC.
There is a very wide range of snow. In particular, the heavy wet stuff acts and behaves very differently from "dry" snow -- whether that's light powder on top, or compacted, but dry, snow underneath.
Wet snow is increadibly heavy, and frequently contains or mixes with ice which makes removal (blowing, ploughing, shoveling, 'dozing) far more challenging.
