Definitely has nothing to do with smaller trucks being more expensive than large ones.

Smaller trucks are cheaper. Looking just at what shows available at my local Ford (I'm sticking with them as a brand for this discussion) their F150s start at 54,000 CAD (4WD, 4 door, 2.7L V6) Their Maveriks start at 40,000 CAD (AWD Hybrid, cloth seats, steel wheels). The cheapest Ranger is 49,000 which is shocking. The little trucks ARE cheaper.

Also nothing to do with regulations forcing larger trucks every year to keep the fleet MPG rating they've been chasing.

I was recently watching a video on youtube about this that cited some research that suggests that the effects of that are massively overstated.

I agree that government should be doing something different to push people to smaller vehicles though.