I drink way too much soda. And never ever the diet stuff- I'll take my chances with the high fructose corn syrup rather than flirt with substitutes like Aspartame, which may be linked to neurological disorders. Total consumption works out to 1-2 liters a day depending on working conditions and ambient temperature. But I am very careful to sip it gradually through the day, as the steady supply of sugar dampens out fluctuations that cause mood swings. Again, I'd rather chance the corn syrup than deal with other chemicals whose effects are not as well known or introduce side effects.
But all that aside, if you plan your meals factoring in the energy from that sugar, the only thing you're really risking is the effects of sugar and insulin. Obesity can be avoided by reducing food intake to balance out the energy from the sugar, and as long as it is sipped instead of slammed you get a nice stable energy burn all day long without the rise and fall of the normal 3 meals a day causing noticeable changes in mood and energy level.
On the obesity front, I believe antibiotics and growth hormones are still almost universally used in animal farming in the States(?)
Nope. Here in New York we're allowed to use antibiotics, and some farms do use different growth hormones such as BST. But these products have a withholding period. After treating the animal with them, any food produced by them must be discarded for a set time period, be it something they produce while alive like milk, or the meat yielded by their slaughter.
There is also strict quality control for food entering the public market:
For instance, when the truck comes to collect the milk produced by a farm, a sample of the holding tank is taken and analyzed to check the quality of the product and ensure it is not contaminated. Any farmer that produces a "hot" tank which fails the quality checks must then pay for the entire truckload at their expense, and the entire truckload is thrown away.
Small farms will often benefit from going organic though, which to maintain the status of means that only bare minimum vaccinations for the animals are allowed, all other veterinarian tasks must rely on natural methods and animals can only be fed using feedstock of organic origin, which is recommended to be grown on-site but can be trucked in from other certified sources.
"Could antibiotics be causing the obesity epidemic?"
Sounds like someone doesn't know what antibiotics are.
An antibiotic is a substance like Penicillin (which is widely used in agriculture as point of fact, I still have a vial of it in the refrigerator) that is able to neutralize bacteria. They are used in humans too for certain types of infections, although the original Penicillin is no longer used in humans due to widespread sensitivity to it and antibiotic resistance effects.
Growth hormones on the other hand could most definitely be bleeding through into humans, and almost certainly are affecting us. They use a shorter hold-down period than antibiotics, allowing for higher concentrations in the end products to reach the consumer. Effects include people being taller and heavier, but also less obvious ones such as internal organs having abnormal sizes or properties. There are some that are also known to alter one's physical appearance given sufficient exposure- which often is cumulative. I have heard that the hormone BST used on the dairy farm is known to bleed through to commercially available milk supplies.