The colon and intestines are surrounded by what is called omental fat. When a dog gets really really fat, the abdomen is occupied by more fat than there is room for intestines. So the intestines may become compressed and their blood flow may be compromised. This can result in occasional diarrhea, even, frequent diarrhea. One cannot assume that this is the pathology, because pancreatitis is extremely common in dogs at this level of obesity.