Can't tell how serious you're being, but given the width of some streets in Japan, the answer is "more than [insert rest-of-world pickup/van here]". A pickup that's too big to get to its destination won't carry as much as a Carry...
Mmkay, but that still doesn't answer the question.
Japan's urban speed limits are around 25mph. The country's national speed limit is about 62mph. And items exist in this world that need moving from place to place that don't require a payload of several tons in order to move them.
I'm sure they don't, but that doesn't stop the vehicle above being ideally suited to the environment for which it was designed.
Seems like the items could fit into a normal car.
The bed seems shallow for carrying much of that, I suppose it has a purpose.
Pretty good for learning to drift in the wet ( the rwd version ) 
or carrying people ( van ) or stuff around town ( pickup )
Around half of the "normal cars" sold in Japan also fit into kei-car regulations, just like this truck.
"Most things". Like what? I doubt people down narrow Japanese side streets have much use for say, large quantities of lumber, or the ability to tow a speedboat. They may have use for small vans delivering local goods however.
I'm abundantly aware you don't care. That doesn't mean it isn't useful.
Just as well your world isn't urban Japan then, isn't it?
You are allowed to have opinions like that, but opinions are only worth listening to when they're based on knowledge.
