Well, it's kindof self-explanatory because constipation accumulates wastes in the tummy, so naturally it increases tummy size. It's no-brainer.
Besides, constipation adds to tummy bloatedness, so that's another cause for tummy size increase. And if waste accumulates in the tummy, more toxins congregate there and that disrupts proper digestion. Poor digestion leads to poor fat storage regulation.
In other words, if your digestive system works improperly, your body tends to store more fat.
Clearly, you have to get rid of constipation.
And the cheapest way is to eat more fresh greens---green leafy vegetables. I don't recommend synthetic anti-constipation supplements or even chemically preserved fruits like preserved prunes. The preserving chemical used can cause allergies.
I do recommend preserved fruits made by proper fermenting.
So, eat more green veggies. And here I recommend moringa (malunggay), spinach and mustard. I especially recommend mustard (mustasa) leaves because they're easily available in wet markets in Manila (and most Asian countries), they're cheap and because you can easily mix them with souped dishes, meat or fish.
I love it in fish sinigang dishes like maya-maya or milkfish (bangus) sauteed in miso.
Green leaves make bowel movement easier and smoother, the natural way. This also causes you to lose weight if done regularly. Sometimes, bowel movement takes effect after a meal of green leafy. Sometimes it takes a day or two, depending on the severity of the constipation.
But don't eat too much if you tend to get bloated. Too much fibers can bloat tummies, too.
Don't worry if the leaves don't seem to have been digested, coming out intact, especially moringa. The human body naturally does not have enzymes that digest green leafy. But the good thing is that green leafy "magnetizes" stubborn stool that refuses to come out during bowel movement. It takes with it clogged wastes so you get rid of constipation.
By the way, below is a video on how to make pickled or fermented mustard leaves (my favorite).
With regular bowel movement due to green leafy, you'd lose tummy size eventually. It's cheap and effective tummy shrinker for cheap health and fitness with middle income budget.
And do avoid carnivorous diets, as much as possible.