What happens to fecal matter if it's continually re-eaten?
In a healthy person, stool weight depends mainly on the quality and quantity of the diet. The mean stool weight of a normal defecation is about 320 g [1].

Dry weight of fecal matter contains [2]:


30 % bacteria
30 % undigested food and fiber
10-20 % fat
2-3% protein


Let's do the math (the only nutrients in feces are fats and proteins):


20% * 320g = 64g of fats which can release 64 * 9.3 = 595 kcal of energy
3% * 320 = 9.6g of proteins which can release 9.6 * 4.1 = 40 kcal of energy


That's about 650 kcal. 2,000 kcal is a rough average of what people eat in a day. But your body might need more or less than 2,000 [3].

As you can see, fecal matter can't supply enough nutrients. Continuously (re)eating feces and only feces will lead to denutrition, less feces and death.

References:


Seyed Mohammad Kazem Hosseini Asl MD, Seyed Davood Hosseini MD. Determination of the Mean Daily Stool Weight, Frequency of Defecation and Bowel Transit Time: Assessment of 1000 Healthy Subjects
shigeta's answer on What is the composition of human feces?
HowStuffWorks.com. How many calories does a person need daily?