Things that wax and wane grow larger and smaller, like the moon. Things that wane simply grow smaller. "My initial enthusiasm for helping waned when I saw the pile of envelopes that needed licking."
You may have heard the expression "on the wane." It means a fad, or a fashion, or a disease, that is on its way out the door. "The face mask fad at school was on the wane as new reports of kids infected with a dreaded new fly virus waned."