If you use a heaver box blade then maybe they feel when you hit something to hard backwards then the 3pt tractor parts will be more prong to brake then the box blade.
Seems logical. Gonna be hard for me to find a 5 ft that comes close to 375 lbs and still is more than light duty. I 'd like 5 ft so it extents a bit outside of rear tires. I wonder who might makes such a thing.
Kubota has historically been very conservative on their implement size recommendations. I guess it only makes good business sense for them to minimize their liabilities. If you are not abusive with your equipment, do a little reading here and it will show you can get by with a bit more than Kubota recommends.
MarkV