I would think the density would be the major factor of weight.
Thats true, if the material is dryer you can load more, there is provisions to add chemicals or water to a load while in transit, it would be a no brainer to have a load of ammonium phosphate dry and have one or two water tanks filled with diesel fuel, just prior to the target the diesel can be introduced into the turning drum while on the hiway, it would take less than 5 minutes to mix it, all that would be left is a detonator charge.
I routinely have to add water to a load to slump it, I do not know the exact formula of AMFO though I do know about thermite, aluminum powder and iron powder or rust.