I dont think soda pop is the only reason for obesity by any means, but I think it is a major contributer for many people and heres why.
There are 3 types of fuel your body uses to make energy. Sugar (glucose), protein, and fat. Of these 3 types, Sugar is the easiest to break down and burn. If you consume large amounts of sugar your body will resort to using sugar as its main source of fuel. Therefore, if you eat a large meal consisting of protien and fats, but wash it down with 3 glasses of soda pop, your body will use the sugar in the pop for fuel, and store the meal as fat. This will result in obesity.
I really dont think it is one thing that causes obesity, and I really dont think that just cutting calories and exercising more is the answer either. I mean sure these methods may work in the long run, but I believe there is an underlying issue. One major problem I see is the consumption of simple carbohydrates along with fats at the same setting. The simple carbs are broken down rapidly and turned into sugar (glucose) which your body uses for fuel, and the fat that is consumed is stored, much like the effect of drinking soda pop. Simple carbs are really nothing more than a highly concentrated form of sugar. Any processed carb in the form of white bread, pasta, cake, cookie, doughnut.....ect. which has been made with refined white flour is just as bad for you as soda pop in my opinion. When these carbs are consumed with highly concentrated fats or large portions of proteins the effects are a prime environment for obesity to occur.
Another point that can be made is the fact that these simple carbs are bleached and put through a bromide process. When the flour is bleached and bromide is added, it kills the germ which holds the majority of natural enzymes and nutrients in the flour, bascially stripping it of any useable nutrition. Bromide is known to inhibit the function of the pituitary gland. The pituitary gland regulates the funciton of every gland in the body which in turn regulates every hormone in the body. This includes the function of the thyroid which regulates metabolism.
This is just a spec of dust compared to the whole underlying issue at hand with processed and refined foods. Alot of the public think that an overwieght person just eats too much and doesnt exercise enough, but what they fail to realize is that the whole commercial food industry is the cause of the rapidly increasing population of obese individuals.
Edit: Some of you may be thinking, well I'm safe cause I drink diet drinks......but contrary to what you may think, these drinks are severly detrimental to your health. Diet drinks or sugar free drinks, use artificial sweeteners and flavorings such as Aspartame, nutrasweet, splenda.....AKA: Excitotoxins. These excitotoxins corrode the receptors of the brain and result in many brain disorders, to name a few: Parkinson's, Alzheimer's, and bi-polar disease.