Earlier quoted context omitted.
Thanks for the links. Why is the phrase "conspiracy theory" used for that though? Sounds like it's a value-judgement free decision from manufacturers to follow the incentives from the policy as we'd predict: way cheaper and less work to make those (now pretty popular) vehicles bigger than R&D and re-tool factories to make smaller and/or more efficient. It would need to be made more efficient to get smaller otherwise…
I was a bit tongue in cheek and playing off the previous comment using the phrase "conspiracy theory". It has a bad connotation, but I guess not all things that sound like conspiracy theories are false. Another car themed topic is when Standard Oil and Firestone Tires colluded to buy up trolly lines and shut them down.
Things can be a conspiracy of common interests (and not even necessarily be bad), whether or not that's what this is, without involving "smoky rooms" (also known as boardrooms; but for fun on that topic, check out full history of NCR, company was nuts).