The weight might be an issue on the front axle, but I think when they refer to loading they are talking about the amount of horsepower being transferred through the front drive train. The components in the front drive train are much smaller than the rear end of the tractor. Thus if the rear wheels are providing very little traction the front end gears take the brunt of the power being used and shorten its life.
We have (I say we, actually my mechanics) been into the front end outter wheels twice on my FIL's JD970 (Yanmar). I think he has about 2500 hours on it. The gears are not very robust, but then it is only around 25 hp.
The reason for both times repair on his was not loading issues though. It was plastic baler twine issues. He has a pasture that is rented out but he has a very small Dearborn manure spreader that he likes to spread manure from the feeding area on the pastures. The neighbor does a lousy job of removing the plastic twine from the hay bales. FIL with very poor eyesight would spin the front wheels in the manure to get a bucket full, wrapping the twine around the wheel and streching it into the bearing seal area. Even if the outter area was cut off, the part that could not be seen eventually takes out the seal and that eventually leads to bearing failure. The first time we had brought the tractor to our shop for a much needed service job and caught it just in time. Oil was almost leaked out and outter bearing bad but new bearings fixed it. The next time it happened not so lucky. A bearing ball dropped down into the gears. It chipped a tooth about a quarter way off but some judisious use of a carbide bur and new bearings and he was going again. Had the ball broke a tooth off, broke upper shaft, or punched a hole in the casing the repair would not have been cheap.
So that is a warning to anyone with these front wheel drives on these small tractors. Don't let baler twine wrap around them as it destroys the seals (actually same goes for rear axles). Also, if you see a leak start for whatever reason, get it repaired. There is only a small amount of oil in these units and what might look like a small leak might mean it is close to running dry. Or at least check all the front end fluid levels more often if there appears a seep or leak.
