I use dd to backup my ubuntu, but the backup image is too large

I use dd to backup my ubuntu, but the backup image is too large

I use the command dd if=/dev/sda | gzip > /media/test/system_backup.img.gz to backup my disk. But the system_bakcup.img.gz is up to 5GB, and when I compress this file, the system_backup.img is only 3.5GB, please help me!