Tom,
The difference is in such things as gearing..... The Tow Boss package for instance, has 4.30 gears, vs the 4.10's that my dually has. I believe the 450 and 550 also have lower gears than your 250 or my 350. Obviously, different gear ratios will cause the engines/trannies to run at different rpms and different levels of "stress" to both get a load moving and keep it moving. To go a little further with that, this tranny guy also told me that running tires with a larger diameter (oversized tires) and lifts will kill the tranny faster than anything else. He says the bigger tires with their increased weight and essentially higher gear ratio, increase the "stress" exponentially. Since changing the tire diameter clearly is changing the effective ratio of the rearends to something higer, it is not much of a stretch to see that putting a lower gear ratio in the Tow Boss package would be a help with the heavier loads it is rated for.