Weight loss in a dream commonly reflects a sense of release -- shedding a burden, a habit, or an old self-image you no longer need to carry with you. It often surfaces after letting go of something significant, marking a lightness you're just beginning to feel in your waking life.
Every beginning when the Moon shall be joined to a Retrograde planet will soon be destroyed, and if she shall be otherwise impedited, the mischief will happen the sooner. - William Lilly (1602.-1681.)