An average snowfall contains about one part water to nine parts air. This is due to the crystalline shape of snowflakes. Snow that has been shoveled or snowblown is a bit more dense, but is still more air than ice. Over time, snow tends to settle under its own weight. Melting and wind can contribute to the settling. This is why the drifts and piles of snow, including those on the roof of your house, are not quite as tall as they were a week ago.
Right after last week's blizzard, many people were claiming they could not remember snow piles so high. Anecdotal memories of such things are often imprecise, and most of those piles have settled some since last weekend, making them appear a little less foreboding regarding the spring flood threat. Of course, the settling does not affect the water content of the snow, it merely looked worse than it was last weekend.