Aston Martin through their copycat philosophy have learned what makes the Mercedes aerodynamics "tick" and thus have been able to integrate the observed Mercedes side-pod tightness and possibly other cooling arrangements into their own design without losing aerodynamic efficiency.
McLaren and Williams on the other hand are ignorant to what boundaries can be pushed with regard to sidepod profile with the Mercedes Engine. One is a newbie and the other has such a mess-up aero-philosophy they can't do anything but run a big fat bulb of a cover around the engine without melting everything in Dali-like fashion.