I beg your pardon Lutz but tyre width among it's other dimensions, including wall thickness, is directly related to it's load carrying capacity and at what pressure
Tyres of a certain construction have a maximum air pressure throughout that range, add to that, the volume or air required to support a given load is a constant.
Therefore the pressure required to get the required volume of air into the smaller volume within a tyre will vary depending on the area contained within any given tyre.
So given a 6ply tyre with a maximum designed pressure of 54psi.
175 being smaller internally than 185 will carry somewhat less load than the 185 at the maximum pressure, or below the maximum 54psi, the 175 will carry the same load but at a slightly higher pressure than the 185.
All tyres of the same size are the same dimensions externally, any changes in construction that effect wall thickness will then change the available volume internally.
Therefore, the same two tyre sizes of the lighter 'car' tyre type with their thinner walls, will carry more load for a given pressure than the 6ply, but only of course up to their specific designed maximum pressure.