I realize that expression has nothing to do with actual fecal matter. But supposing a person was constipated for a while, and hadn’t been faithfully making his offerings to the porcelain god—just how full of it is “full of it”? How much poo are we talking about? Pounds? Liters?
Megacolon image search. Not for the weak of stomach.
Mutter Museum display. If I remember correctly-40 lbs.