So, cold launched missiles need a concentric tube due to the gas piston mechanism and all the gas pressure building up below the missile. Basically you have the missile, which may be round in cross section, but due to its folded fins, its more like a square or a hexagon in cross section, depending on how you fold those fins. There's some space in between the missile and the cell walls, due to those folded fins. Then you have a circular cross section of the container. and all that then has to fit into the square of the missile cell.
Hot launched missiles don't need concentric tubes, but they need room for dedicated exhaust vents around the missile, which again eats into the space around the missile body available.
Any way you slice it, if the missile cell is, for example, 850mm wide, actual width of the missile body is not gonna be over 650mm or so mm.