[DSY011]

I think that the U.S. trains were bigger and heaver than U.K. trains because they had much longer distances and heaver loads to deal with. They also had to travel further between water stops. Their coal and water tenders had to be bigger. Like the loco's of the Rhodesian Railways, who had large coal and water tenders who also had to travel long distances before they could refill. They both also had fewer goods trains running, so carried a lot more goods to small towns that were far from the big city's.
U.K. trains did not have too far to go before they could refuel, therefore the loco's did not have to be very large. Also there are not many areas of the U.K. that did not have A train with in a short distance from a station in the hay-day of the British railways. Sorry to say, most goods now travel by road in the U.K.
Hope this is a start in answering your question.