You will be able to eat more and more as you progress through the first few months post-op. This isn't unusual and it's nothing to panic about. My nut says I can eat a cup of food at a time now that I'm 5 months out, and generally, she's right. I couldn't eat a cup of dense Protein like chicken, but I can eat a cup of Soup or yogurt, which pretty much go right through.
With regard to stretching - personally (and based on what I've read/seen) I don't think it's stretching as much as it is eating around the sleeve. Imagine my surprise when I decided to have "a few" potato chips. I counted out a serving, tracked it, and was not even a little affected by the intake. I could have eaten the whole bag! You've heard of slider foods - well, believe me, they are a thing. Because sliders don't close the pyloric valve they slide right through and your capacity is much higher. Obviously this increases calorie intake and voila - weight gain ensues.
One rule I don't break is drinking when I eat. This will also cause you to eat more than you should because it pushes food through the sleeve allowing you to accomodate more volume.
I'm not perfect - I eat small pieces of chocolate here and there, I have popcorn and chips once in a great while, sometimes I'll have a small piece of bread with butter. 95% of the time, though, I follow my plan and I measure and track everything in My Fitness Pal (even the popcorn and chocolate!) That, to me, is the key - being aware of everything that I'm eating. Presurgery I ate with abandon, and that's never going to be the case again.