There's no such thing as a "squatting toilet". You are crouching and shitting on the ground like an animal, only there happens to be a sewer drain under your ass instead of the more usual soil.
Incomplete evacuation is caused by colon inflammation as a result of chronic irritation. Overeating, too much booze, low-quality or rancid foods cooked in gutter grease (spiciness often masks rancidity and compounds irritation), too much laying around, high stress levels during your normal shitting time...all of these irritate and disrupt the functionality of the lower gut.
>compress the abdomen with the thighs
How fucking fat do you have to be that squatting compresses your abdomen?
Obviously you've never spent time in the woods, JFL. It's a royal PITA to shit on the ground when you have a sluggish bowel for whatever reason. In any case, brownoids seem to experience constipation at a lower rate than whites do.