I think dyno load is constant for all LDVs.
Emissions limits are determined by
vehicle weight and age, not gearing AFAIK. In order to compensate for heavier vehicles having to move around more weight, their pass/fail limit is lower than a lighter vehicle's would be, so that all vehicles on the road stay under whatever level of emissions per mile. I'm thinking it's a lot easier to just change the emissions limits than it would be to have an adjustable dyno.