A lot of the off the peg bag jackets will buldge out on the sides, where the side vents would be, when they are worn open; this is minimalized on one that fits well, though. I noticed this while experimenting with kilt jacket conversions, and decided to try putting pleats along the back-side seams to take up that spare material. The pleats are sewn down in the waist area, creating darts toward the shoulder blades, and the back flap below.

The kilt jacket can't really be closed after this alteration, but it is supposed to always be worn open, anyway. Some day, I will have to try to get pictures and post them, just to see if it looks all right, but the back of that jacket feels like it fits correctly. I wish I had a suit jacket that fit like that, and could be closed in the front, of course,