The purpose of the stomach is mostly to use acid and enzymes to break the chewed food into a mushy sludge that can be processed in the small intestine. Pretty much all the stomach does is take food in, store it for a while, produce acid and enzymes to make food sludge, and then slowly feed the sludge into your small intestine.
Water (along with most of the food nutrients that you consume) is mostly absorbed by the inner lining of the small intestine. That organ is actually quite large, with a massive inner surface area. The intestine is typically more than 6 meters long, with a diameter of 3-5cm. That means that the small intestine can not only take in a large volume of liquid, it can also absorb the liquid fairly quickly. The water and nutrients absorbed in the small intestines go directly into the bloodstream to be processed and used by the various organs.
In a GI tract that has not been modified by surgery, the food and water can sit in the stomach for 2-3 hours before being completely emptied into the small intestine. In your case, the water may be going almost directly into your small intestine, so long as there is no food to hold the process up. (This depends on the details of your surgery.)
It's completely reasonable that you are able to consume more water than you might think (given your food intake). Your surgery may have modified your upper GI tract in a way that makes it actually more efficient than it was before at taking in and absorbing water.