I eat, what I consider, a fairly balanced diet. I am, as a southerner, big on vegetables. So what gives on the size of the ‘ahem,’ loaf?
What in the world am I disposing of that takes that size?
Have you ever seen the pile of ashes left over from a fire? All the stuff that didn’t burn up.
Same thing with food – all the good stuff (whatever the body can make use of) gets pulled out of the food, and the left over stuff goes on through.
In its passage some fluids and compaction by muscles give the charactistic shape and size.
Slightly tangent, but still relevant: [Why is, um, fecal matter brown? - The Straight Dope](Why is, um, fecal matter brown?)
By weight, something like 2/3 is water.
For the dry solids, stool is around 1/3 bacteria, 1/3 relatively undigested fiber, a fair amount of fat (10% or more) which doesn’t digest easily, and some inorganic material that came in with the food. Plus an assortment of other crap 
.
Your mileage does vary, as everyone eats different foods and has different bowel habits. Also, the water weight varies remarkably from person to person.
Here’s the thing. What we call food is not really food. It contains food. It contains the nutrients, some of them micro-nutrients, that we need to absorb in order to survive and keep the system humming along. Maybe a couple of mg of this, and a couple of mg of that. Or whatever, — sometimes measured in grams (protein, carbohydrate), but nowhere near the actual weight of what is consumed is absorbed.
If you actually weighed what is on your plate 3X a day + snacks, you come to a hefty amount. Thus we are left with a heap of, I don’t know, let’s call it “organic waste product” that needs to somehow be eliminated from the organism.
The body is like a factory… raw materials entering on one end and toxic waste 
exiting from the other end.
Correct link (URL was correct but link coding was not)
There is actually an index entry in one of Cecil’s early books (I’ve been reading Cecil since before many of the posters to this forum were born):
Shit, why brown
the loaf issue is because you’re relaxed or have good muscle control. you could pinch it if you want, that would take muscle control and practice.
keep a food diary, including water volume. practice, report results.
Not quite a serious answer, but it’s (along with its liquid counterpart and those closely associated gases) the only consistent and universal output of the human race. Other animals contribute as well. Plants?
From my conversations with plants, I hear that there is a definite exchange of substances both above and below ground.
After all the plants eat shit and shit food.
Question answered.
Exceptionally good book on this, if you can find it. I’ve recommended it on this Board before
Dan Sabbath
End Product: The First Taboo
So crap is made out of crap! Whoda thunk?
I’ve heard that the dead corpses intestinal bacteria constitutes a major proportion of the total bulk.
Do plants excrete in any way? They exhale and have sap, but I don’t know of any regular poop-like activity.
Fallen leaves could be regarded in that way, I guess, but most of what plants excrete is CO2 (from respiration).
And as I was told by a wise elder, turds are pointed at one end to prevent your asshole from slamming shut.
SOME of the good stuff.
Some of the total energy.
Some of the micro-nutrients. There’s always been questions… yes there is iron in spinach… can you absorb it , or does it just pass straight through ? They say its not really a good source of iron.
Gulp by Mary Roach answers this - and many adjacent questions - admirably. I highly recommend all of her books.
Wombats poop cubes. I imagine they’re quite visible.
As I said, they eat food from under the ground and their turds are consumed by humans and animals as “food”.
I don’t know if it’s fair to consider a plant’s fruit or seeds a waste product. I don’t know about you, but I ain’t shittin’ babies.
(Shittin’ to the Oldies, maybe, but not shittin’ babies.)