Jim chugged the soda down, but it wasn't easing his craving for a drink, it was compounding it. The liquid and gas from the soda settled into his stomach, making him feel full, but he couldn't stop drinking. After the first one was finished, Jim took a second and started drinking madly.
The contact that he'd had with the donkey pool toy had begun to change Jim, though he hadn't noticed yet. While he felt full, Jim didn't realize that the soda was expanding his belly outward like a balloon, allowing for more of the soda.
After a third bottle of soda, Jim had a massively round belly, sagging with the weight of the liquid. It almost looked like he had swallowed a beachball.