Kubota B and L tractors are optimized for lighter weight to improve finish mowing. That means lighter front axles which in turn means lower rated FEL and therefore bigger front tires are not necessary.
Check out similar HP CUTs from Mahindra and Kioti who do not try to build lighter weight tractors. For example Kubota
L3940 weighs about 500lbs less than Kioti DK40se or Mahindra 4540. You see the same thing with MX style which is a smallish utility style which still weighs a few hundred pounds less than equivalent Kioti DK.
I don't know if it is an absolute rule but generally tractors in same HP range that have higher loader lift capacities will have bigger front axles and bigger front tires.