I'm the exact same way. In fact I was discussing this disturbing fact with a buddy the other day. I'm a monster form the side, but straight up I have no chest or shoulder width. From the back I'm wide as I have a wide back, but that can only really be seen from the front during a front lat spread. You lack chest and shoulder width. IT'S GENETIC. From the side I can rival anyones side chest pose and I have tree trunks running down both sides of my spine. This is all thickness. But, I lack frontal width just as you described. I have very narrow pecs and flat medial delts. You can do all the lateral raises you want and do flyes until you grow wings, but genetics has the final say in this one.