I'm not sure if it's really true. In any case, the sit bones are the points that carry most of the weight that rests on the saddle, and their width is fixed anyway. In my experience, wider saddles can be comfier for shorter rides, especially if your bum hasn't gotten used to long hours in the saddle. But on longer and faster rides a wider saddle can attribute to excess rubbing and chaffing in the groin area.