Often, pieces of feces have a clearly-defined shape and size — both length and width (that is, cross-sectional diameter, or caliber). But that width seems to vary quite a bit, even among pieces of feces from the same person. Why? What determines the width? Why is it wide when it's wide, and why is it narrow when it's narrow?
With the help of Bristol Stool Chart (BSC) and full explanation of conditions related to different stool sizes from this link you could have a full understanding over stool size.
» Type 1: Separate hard lumps, like nuts Typical for acute disbacteriosis. These stools lack a normal amorphous quality, because bacteria are missing and there is nothing to retain water. The lumps are hard and abrasive, the typical diameter ranges from 1 to 2 cm (0.4–0.8”), and they're painful to pass, because the lumps are hard and scratchy. There is a high likelihood of anorectal bleeding from mechanical laceration of the anal canal. Typical for post-antibiotic treatments and for people attempting fiber-free (low-carb) diets. Flatulence isn't likely, because fermentation of fiber isn't taking place.
» Type 2: Sausage-like but lumpy Represents a combination of Type 1 stools impacted into a single mass and lumped together by fiber components and some bacteria. Typical for organic constipation. The diameter is 3 to 4 cm (1.2–1.6”). This type is the most destructive by far because its size is near or exceeds the maximum opening of the anal canal‘s aperture (3.5 cm). It‘s bound to cause extreme straining during elimination, and most likely to cause anal canal laceration, hemorrhoidal prolapse, or diverticulosis. To attain this form, the stools must be in the colon for at least several weeks instead of the normal 72 hours. Anorectal pain, hemorrhoidal disease, anal fissures, withholding or delaying of defecation, and a history of chronic constipation are the most likely causes. Minor flatulence is probable. A person experiencing these stools is most likely to suffer from irritable bowel syndrome because of continuous pressure of large stools on the intestinal walls. The possibility of obstruction of the small intestine is high, because the large intestine is filled to capacity with stools. Adding supplemental fiber to expel these stools is dangerous, because the expanded fiber has no place to go, and may cause hernia, obstruction, or perforation of the small and large intestine alike.
» Type 3: Like a sausage but with cracks in the surface This form has all of the characteristics of Type 2 stools, but the transit time is faster, between one and two weeks. Typical for latent constipation. The diameter is 2 to 3.5 cm (0.8–1.4”). Irritable bowel syndrome is likely. Flatulence is minor, because of disbacteriosis. The fact that it hasn't became as enlarged as Type 2 suggests that the defecations are regular. Straining is required. All of the adverse effects typical for Type 2 stools are likely for type 3, especially the rapid deterioration of hemorrhoidal disease.
» Type 4: Like a sausage or snake, smooth and soft This form is normal for someone defecating once daily. The diameter is 1 to 2 cm (0.4–0.8”). The larger diameter suggests a longer transit time or a large amount of dietary fiber in the diet.
» Type 5: Soft blobs with clear-cut edges I consider this form ideal. It is typical for a person who has stools twice or three times daily, after major meals. The diameter is 1 to 1.5 cm (0.4–0.6”).
» Type 6: Fluffy pieces with ragged edges, a mushy stool This form is close to the margins of comfort in several respects. First, it may be difficult to control the urge, especially when you don‘t have immediate access to a bathroom. Second, it is a rather messy affair to manage with toilet paper alone, unless you have access to a flexible shower or bidet. Otherwise, I consider it borderline normal. These kind of stools may suggest a slightly hyperactive colon (fast motility), excess dietary potassium, or sudden dehydration or spike in blood pressure related to stress (both cause the rapid release of water and potassium from blood plasma into the intestinal cavity). It can also indicate a hypersensitive personality prone to stress, too many spices, drinking water with a high mineral content, or the use of osmotic (mineral salts) laxatives.
» Type 7: Watery, no solid pieces This, of course, is diarrhea, a subject outside the scope of this chapter with just one important and notable exception—so-called paradoxical diarrhea. It‘s typical for people (especially young children and infirm or convalescing adults) affected by fecal impaction—a condition that follows or accompanies type 1 stools. During paradoxical diarrhea the liquid contents of the small intestine (up to 1.5–2 liters/quarts daily) have no place to go but down, because the large intestine is stuffed with impacted stools throughout its entire length. Some water gets absorbed, the rest accumulates in the rectum. The reason this type of diarrhea is called paradoxical is not because its nature isn‘t known or understood, but because being severely constipated and experiencing diarrhea all at once, is, indeed, a paradoxical situation. Unfortunately, it‘s all too common.
Interestingly, the interpretations and explanations of the BSF scale that accompany the original chart differ from my analysis. To this I can only say: thanks for great pictures, but, no thanks for the rest...
Excerpted from Fiber Menace, page 117-120; BSF Chart: wikipedia.org
But to make it more clear:
Reasons behind thicker and harder stool:
Lower fiber food
Lower liquid intake
Lower physical activity
Lack or lower amount of bacteria related to retaining water inside rectum and colon
Many medications have constipation as a side effect. Some include (but are not limited to) opioids also known as narcotics, diuretics, antidepressants, antihistamines, antispasmodics, anticonvulsants, and aluminum antacids. Certain calcium channel blockers such as nifedipine and verapamil can cause severe constipation due to dysfunction of motility in the rectosigmoid colon.
Metabolic and muscular
Metabolic and endocrine problems which may lead to constipation include: hypercalcemia, hypothyroidism, diabetes mellitus, cystic fibrosis, and celiac disease. Constipation is also common in individuals with muscular and myotonic dystrophy.
Structural and functional abnormalities
Any physical digestive tract line problem which cause interference of stool movement. Like cancer or some kind of inflammation that could ban the canal
Constipation has a number of structural (mechanical, morphological, anatomical) causes, including: spinal cord lesions, Parkinson, colon cancer, anal fissures, proctitis, and pelvic floor dysfunction.
Constipation also has functional (neurological) causes, including anismus, descending perineum syndrome, and Hirschsprung's disease. In infants, Hirschsprung's disease is the most common medical disorder associated with constipation. Anismus occurs in a small minority of persons with chronic constipation or obstructed defecation.
Voluntary withholding of the stool is a common cause of constipation.The choice to withhold can be due to factors such as fear of pain, fear of public restrooms, or laziness. When a child holds in the stool a combination of encouragement, fluids, fiber, and laxatives may be useful to overcome the problem.
In the second provided link you could get how cross-sectional diameter changes based on what you eat or how you behave with your body mentally or physically if you are a healthy person.
But as you are interested more on cross-sectional size of stool and reason behind it, consider the fact that colons and rectum are water absorber. The more stool stays there the more thick and dry it gets. So if you ate more fiber you will probably have softer stool and thinner caliber as the result of pressure of the rectum canal. Because some bacteria which are active in the fibers of stool are water absorbers too and the reason behind why doctors insist on eating more probiotic products is to keep these bacteria present and stable and also to get vitamins from their activities. So you could change your caliber only by eating more fiber more water and having more physical activities and also for consistent thick and dry stool (constipation) individual must share the problem with doctor because it could bring more dangerous problems in future or maybe it's a sign of some other serious disease that person's body is already dealing with. Most of the time it's nothing but our diet and hours we are sitting for variety of reasons or maybe focusing on another activitys (work, computer games, disabilities, watching tv and so on and so forth).