The wings make the caps easier to handle in combat and also provide a reference as to which end goes on the nipple without looking. They are also easier to handle in the cold. There's no significance to the number of wings and most likely was chosen as the easiest way to split the cap when manufacturing. Small caps (#s 10, 11, 12) existed very early in the percussion era and their size has no bearing on the compound they contain. Musket caps were developed to meet the military's needs and yes they are stronger, most likely to provide ignition under any and all combat conditions.
