Looks to be axle, etc
Rated at 2800kg, stock tyres are 124 rated (1600kg each), so it isn't the tyres that is determining the rating.
The Hino 817 front axle is also rated at 2800kg (but with an extra 1000kg GVM) and the Isuzu is 3100kg, so none of the Japanese trucks have a very high front axle...