James asked a fair question.
Before I respond, I'll have to preface it with the following
drivel disclaimer. My reply does not represent the thoughts of my employer, nor is this piece of real estate endorsed by my current employer.
To be clear, my expertise is actually Powertrain subsystems, not Body Interior. The irony, is that recently I spent a short 4month stint, working within the Seat organization. These days I fancy myself as a 6-Sigma Black Belt, within the Body Organization. Clearly, I am not the most qualified individual to answer McGovern's question.
However, I would venture to guess that the seats have well-defined constraints (ie H-point, bolster, etc.) Some of the contraints are driven by government and package requirements. For instance, the center console is designed to outfit cupholders (yes the dreaded cupholders), CDs, ladies accessories. The center console also doubles as an armrest in most vehicles. Imagine that many Americans want to cram as much junk in the center console. It would seem logical the center console has increased in size of the years. Hence you have a design or package constraint.
As mentioned earlier, seats must meet rigorous government safety requirements. Restraints (ie seat belts, head restraints-help prevent nasty whiplash) is serious business. Making the seats substantially wider to satisfice obese occupants would be an egregious departure from sound engineering principles. Let's call it Intelligent Design.
The general rule of thumb is that we design for the 5% - 95% percentile human.
Typically that 5% percentile represents the 6'.x male and the 5'.x female. I would imagine these requirements do consider weight too. I am not clear about the weight constraints.
I suppose if obese drivers are looking for comfortable rides, they should consider a large SUV. This would be there best chance at comfort. The problem with this particular choice is sky-rocketing fuel prices. So it would appear that more obese people are sacrificing comfort for cost savings.. Can't please everyone.