Is it common for an underweight person to experience symptoms of constipation if they eat tiny meals?

Yes: The basic concern is why the person is underweight. If they are having symptoms with eating, they should see a physician. When you don't eat well, the bowels are affected. Treating the constipation with high fiber and fluids, prunes or stool softeners, is a short term measure. The most important thing is to address the underlying problem of poor eating and underweight status with your doctor.