Yeah, I'm not overweight, but nor am I underweight. I eat more calorie-dense foods than I used to, I guess.
As far as I know it's better for the j-pouch if you keep your weight optimal and intra-abdominal fat low. I donated a kidney (back before I got sick with ulcerative colitis) and my surgeon said it's best not to gain weight after kidney donation either. Less chance for hernia, type2 diabetes, etc.