I don’t pretend to know all the answers, but I would theorize that your stool size has not increased because the body is just taking in and retaining what it has been needing for a while now.
I doubt the constipation is causing your weight gain. You would be gaining weight even if you didn’t have it, I would imagine, just like how I did and I had no constipation issues. I think if you increased your calorie intake even more, you would finally start getting decent stool volume.
In terms of weight gain, I went from 57kg to 82kg in two months. Now 9 months later, I am 96kg. I haven’t had constipation during any of this.