I agree that people with larger rears probably shouldn't wear side vents, unless they are very carefully tailored.
Actually, I think double vented is more flattering for a big backside. A single vent should hang down dead straight like a plumb line. You close the jacket and let us say, your rear pushes the single vent two inches apart. The vent being two inches out of the true is clearly visible. Go now for side vents: the two inches become one inch on either side. Adding to this the fact that the sides of a double vent are cut at an angle, any distortion from the true is more difficult to detect. Of course it is a question of degree. Your butt might be so big that it pushes out regardless, but than the jacket is a very bad fit and miles too tight around the waist.
No vent at all is often the best choice for those with more ample asses.
Again, a question of degree. If your butt pulls the fabric all over the place, that's not a pretty sight.