Pets become overweight for very simple reason. they eat more calories than what they can spend. Owners are responsible to ensure and regulate proper calorie intake and expenditure.
Factors contributing to obesity are-
Food type: Feeding carb rich and excessive fatty food has cumulative effect leading to obesity.
Neutering - calorie requirements reduce post neutering as the metabolic rate lowers. controlled intake and regular activity helps to keep obesity at bay.
Diseases - Hypothyroidism, Cushing's Disease (excess intrinsic steroids), diabetes etc

Adverse effects of obesity:
Early onset of osteoarthritis/joint disease
Breathing difficulty in short snouted breeds like pugs
Heart disease/high blood pressure
Pancreatitis/hepatitis in cats

Obesity in Pets is completely preventable disease. Cost of managing a obese pet can be financially as well as emotionally taxing. Its always better to prevent than cure!!!!