I'm not getting it. Meat is hard to digest, but you can do it with a short gut, and produces very little excretion (the military "low residue diet" is meaty and low in fibre)

But vegetables are easy, yet take a longer gut and produce enormous amounts of shit

There's nothing about difficulty in digestion on https://en.wikipedia.org/wiki/Meat#Nutritional_information

Have you got a source - ideally one not produced by a vegan or vegetarian source?