I agree with James. There can be some temporary stretching of the RNY pouch or sleeve if you over eat, but the idea that the pouch or sleeve can be permanently stretched out is urban legend based on outdated theories. What does happen is that over time, your hunger can return (this can take 5 years to happen with RNY and 3 years or so with Sleeve). The stomach capacity, however, remains where it was at the time of surgery and you still get full with small portions. People can then defeat the benefits of surgery by eating bad foods more frequently, slider foods (milk shakes, potato chips etc.) or by over eating (which ends up resulting in stacking food in your esophagus--which is bad for many reasons). Surgery is just a tool so if you don't use the first couple of years post-op to change your bad lifestyle habits, weight regain is possible (even likely).
As for the bounce, that's a normal part of the process. The surgery changes your set point weight, so your body thinks it's suppose to weight 190 pounds instead of 300. This is called the set point theory--which is how your body maintains a certain weight. When you're on your way down the scale post up, you'll likely drop below your new set point if you follow the new dietary guidelines so you might hit 170 or 180 at your lowest. You'll then gain some weight back and stabilize at your new set point (190). In truth, if you follow the guidelines and do your part, you probably aren't "regaining" weight in the first year or two post op. Your body is just adjusting for your new set point. If you don't follow the guidelines and you eat the the wrong foods, your weight game can be genuine and that's issue. If you're working with a nutritionist (which will be required as part of post-op care), you'll know if you're going out of bounds with the food. If you have RNY, you'll probably get that info every time you have a dumping syndrome episode.