It has much more to do with the "weight" of the rails. How heavy is it per foot or yard or meter? That and the roadbed determine the "Axile Load". Most Asian Rails were pretty light weight compared to European of American Rails..., often less than half. Which meant the rolling stock had to be smaller and lighter, and also the engines.
This is quite true. And Janes lists the weight of rail for every line in the world - including the secondary spur lines and tirtiary lines. The main lines get more. Japan itself uses a narrow gage - but there is a vast difference between a primary and a secondary line. I don't carry this around in my head - but I can give you both the weight of rail and the weight of loads as measured by rolling stock if you wish. The point is that saying "narrow gage" does not mean it has a low loading. And the main lines used here were of British and French origin, not Japanese. Even the lines laid by Japan in SE Asia during WWII used Malay track.
Malaya: Main Lines: 40 kg and 60 kg per meter. Rails 40 feet long (12.2 meters). Max curve 12.25 degrees. Ruling gradent 1% slope except Taiping Pass 1.25%. Max axilload 16 tons.
Thailand: Heaviest line: 40 kg per meter. Lightest line 25 kg per meter. Many variations between these values for spurs depending on requirements (30, 35, 37, even 42 - 2 different 35s!). Max axelload 15 tons.
Vietnam: uniform 43 kg per meter. Max axelload 14 tons.
Burma: Main line 75 pound/yard rail (= 31 kg/meter). Secondary lines 60 pound/yard (25 kg/meter). Max axelload 12 tons.
