The width of the seat isn't based off the widest part of your body (Well, most people's bodies).
Seat width is based off the size of hips. Which makes a certain logical sense. But for some of us (specifically those with proto-Germanic genes), our shoulders are wider than our hips. When you try to do three wide on an airplane with three broad shouldered people, it freaking sucks. Add on top of this the fact that the backrests are WAY too short (the top of the seat doesn't even come up to my neck), and the cushions are also too short (no thigh support, and God help you if you slouch).
Damn the airline industry for catering to the whims of people who fly once a year and will put up with this crap for the sake of lower fares. For those of us who fly even semi-regularly (I've done three trips to El Paso in Feb alone, and lined up for more), we'd gladly pay more for, let see, AN ACTUAL COMFORTABLE SEAT! But remember that first is 5x the price, but you don't get 5x the service.
I'd take a 1.5x increase in price, just to get a usable seat. But wait, I already AM paying more for my ticket because I have to book at the last minute. (Example: A ticket in advance on Delta to El Paso is $560 roundtrip. My last round trip was $1400 because I had to book the day before) So I pay more to start with than the vacationers, and I get the same crappy service, same crappy seats, and I have even more fight to get the overhead bins because I cannot charge my client to buy more clothes should Delta be incompetent and lose my bag. (Oh Delta lost your bag? Why didn't you cram a suit into your laptop carryon?)
I'd still put up with the crappy service in Coach (oh, sorry, Economy class), as long as I could fly from Atlanta to El Paso without my limbs falling asleep. I recommend a study of the seats in my E-Class.
Delta, I'm ready when you are.