Most peacoats are cut boxy. I had a similar issue about six months ago and had my peacoat tailored to suppress the waist.
Assuming the coat fits you properly in the shoulders and chest, suppressing the waist can improve the fit considerably.
Waist suppression in a peacoat involves taking the coat in along the back seams and the side seams. My coat has three back seams and the tailor took a little out of each to narrow the waist. He did not alter the side seams.