A healthy adult male human body contains roughly 126,000 dietary calories from a cannibalistic perspective. Divide that by the 3,500 cals/lb delta needed to gain or lose weight, and you get 36lbs of actual nutritive mass. A healthy adult male is around 155lbs. Dividing 36lbs nutritive mass by 155lbs actual mass, you get around 23% nutrition by mass, which for argument's sake it isn't a stretch to round up to 25%. Assuming this ratio is similar across the species, and not unique to adult males, this means, if your BMR could be magically reduced to 0 for a day, every pound you pack on from eating a human being would be punctuated with 3lbs of shit and piss. By the time you could gain the full 36lbs from the adult male specimen, you would, by contrast, have shat over 100lbs of ex-flesh out your fat stinky ass.