it's more to do with the weight of the car; but heavier cars tend to have bigger tyres
I'm not sure how to calculate the ideal pressure if you change the tyre size though (On a SUV car that came with different rim size options there were different pressure figures listed for 16", 17", 18" and 19" rims; can't remember if they increased or decreased with rim size)
if manufacturers can recommend different oil viscosities for different climates, fit the car with different radio frequency ranges for different spectrums, would it be hard for them to give different recommendations based on road conditions? the fact that they don't do it implies that the road condition doesn't matter for the ideal recommendation (and other countries have crap roads too).
they do (at least on some cars) recommend different pressures based on the car load, so it means they expect you to pump more if you will carry a heavy load... they could as well suggest a lower pressure when driving on shitty roads, but they don't!
of course it will be uncomfortable with potholes and speedbumps, but you could try slowing down rather than under-inflating tyres... your choice
manufacturers have to consider many factors when recommending optimum pressure, not just comfort: tyre wear, fuel economy, cornering & braking response, and last but not least, the chance of a blowout due to overheating. that risk is compounded by the fact that most people are not aware of speed ratings on tyres.
many people do underinflate tyres, either knowingly (like you), or unknowingly (because tyre kade man recommends it and they think that's the ideal pressure); but that doesn't mean it's a good idea.