Adding oil and water doubles this weight, making it 20-30% of the bird's healthy body weight. Picture 30 one pound boxes of dry pasta and then add water. This is proportionally how much a 150 pound human would be force fed using this formula.
A duck's liver naturally weighs around 50 grams. However, to qualify as foie gras, the industry's own regulations require ducks' livers to weigh an absolute minimum of 300 grams.