A thin person can have GERD, too, but it is less likely. In studies that looked at obesity alone or at genetic risk factors for GERD, being overweight clearly was a major problem, and appears to be a risk factor leading to GERD.
How does overweight cause or contribute to acid reflux? One reason is that gravity and the extra pounds in the body place ore stress on the digestive system. You might assume that gravity would cause food to go down rather than up. More weight would seem to mean more gravity - or so one would think. But more weight causes increased bulk in the upper abdomen and greater difficulty in the esophagus getting the food into the stomach fast and keeping it there. Similarly, extra pressure on the stomach prevents food from being efficiently propelled into the small intestine, causing a backup of the system.
Heavier people are also more prine to hiatal hernia. A hiatal hernia is a condition in which part of your upper stomach moves through the diaphragm and into the chest. This condition weakens the antireflux barriers and may cause retention of acidic fluid or food in the hernia pouchm which is then backed up into the esophagus.