Solid waste tends to be heavier and therefore less volume can be carried.  In some areas, there are limitations as to the weights that can be transported which explains why for solid waste bins are in general smaller.  Solid waste includes:

Although these also make part of the mixed waste, the weight of these is why WastePro has some of the bins available as flat rate and by the weight. If too many of these are included the bin with the mixed waste, it can exceed the weight limit of the bin.