Nope, you did not gain a pound! It takes 3500 excess calories to gain a pound, and I’m 100% sure you did not eat that much a week after surgery.
It’s completely normal for your weight to fluctuate by a couple of pounds. Even over the course of a single day, your weight can fluctuate by more than a pound, depending on the time of day, the timing with respect to eating and going to the bathroom, salt intake (which can make you retain water), etc. A lot of people recommend only weighing yourself once per week so you don’t see those little day-to-day fluctuations.
Mashed potatoes are not the best thing to eat because they’re high in carbs and low in protein, but a lot of programs allow them in the puréed stage (you might want to mix in some unflavored protein powder to add some protein). Is a deviled egg really allowed in the puréed stage? At this point, the concern is not about stretching your stomach so much as tearing the sutures that are holding your stomach together. It’s really important to follow your surgeon’s instructions to allow your stomach to heal properly.