I just made an image of a freshly installed dual boot (Ubuntu and Windows) using this command (which I've been using for a while for smaller images):
dd if=/dev/sda | gzip > /mnt/drive.img.gz
On this drive less than 60G out of 500G are used. Nevertheless that image-file now is 409G big.
How is that? Shouldn't gzip manage to compress all those zeros? As I said, it is a freshly installed system. It couldn't be that cluttered.
Now I didn't expect for the file to be 60G, but 400G seems very huge to me.