Exsept its not that simple, due to poorly conceived emmisons law (one.exempting larger vehicles from the standard) and an unfortunate tariff (see chicken tax) its better for the manufacturer to make ever larger vehicles to use the "small truck" loophole. Its hard to signal a size preference when they are all behemoths due to engineers finding a way to exploit the regulations by making the all cheaper when they are huge.

It is that simple. There is no market pressure to be small, quite the opposite, so that's what happens. They keep getting bigger because, again, consumers tend to choose the bigger option.

Regulations do not push for ever bigger trucks, they may have thresholds so at most they might favour a minimum size but after that it's up to the market. My understanding is that the rules for light trucks date from the 70s, which I'm sure means that trucks could be much smaller and still use that "loophole".

Exsept do to CAFE emissions standards there is a selection presure removing small trucks before market pressure can influence anything.

So so one buys small trucks because there aren't any so people that just need a truck buy are large one as there are no new small truck in the market so sales numbers of large truck go up so more are made because large truck sales are up and viscous cycle continues untilled it gets so bad people who just want a small truck start importing used decades old kie trucks from Japan because regulations have distorted the local market. If it was functioning properly no one would go to all the effort as that is a signal of high unmet demand.

Your comment repeats what I wrote before but does not explain the key point, which that trucks get bigger and bigger, especially since cars are also getting bigger and bigger (at least here in Europe).

That does not happen because of regulations but because, again, that what consumers freely choose when given the choice.