The smaller truck probably carries more in loads than 90% of all pickups on the roads unfortunately. They're not being used like they're designed to be. Or they're being used exactly how they're designed to be I guess.
Okay then how about something truck owners do a lot. Which one is better for a husband wife and two kids? Which one can reach highway speeds? Which one won't make you deaf trying to drive to work?
The Ford Fiesta, a subcompact, can tow 2,000lb. For the vast, vast majority of tow jobs a standard car with a hitch will be just fine. Fuck, half of England tows RVs across their country every summer and they mostly drive cars smaller than Americans.
Oh, and the person driving that Kei truck? He likely doesn't own it, it's for his job. Trucks are work vehicles. Almost no Americans need pickup trucks. It's a luxury status symbol that's highly dangerous. The few times someone needs to haul shit, you can rent one and then you don't have to worry as much about scratching the pristine bed that 99% of American truck owners never use.
Kei cars/trucks/vans are limited to the following requirements to be categorized as such per Japanese regulations:
Length − up to 11.2 ft
Width − up to 4.9 ft
Height − up to 6.6 ft
Engine capacity − up to 660 ccs
Power − up to 63 hp
Capacity − up to 4 passenger seats
Load-carrying capacity − should not exceed 771.6 pounds
1200 is technically possible. But it's past designed brake, suspension, transmission capacity of any Kei truck. I'd urge you to use caution if ever doing it again. Wouldn't go over 20 mph if you can even reach it at that weight.
I definitely don’t disagree with you on transmission and breaking. You can do 35 with 1200lbs of load but a Toyota Hilux will do a better job and I would not drive a Kai truck on the highway regardless of what’s in the bed, your knees are a few inches of thin sheet steel construction from the outside world.