Today’s punch line derives from the belief that the soul has mass and at the time of death (or sale to a demon) the post-mortem (or post-sale) body when weighed will show a slight loss of weight when compared to the pre-mortem (or pre-sale) weight. According to limited and suspect experiments conducted in 1907, the weight was supposed to be 21 grams. The same as the title of the 2003 film, 21 Grams.
21 grams is about 3/4 of an ounce. Not a lot of weight loss. You’d probably lose more weight if you just switched from grande to a tall latte.