Why does a Lower Concentration of SO2 call for an increase of equipment dimensions in a Contact Process Plant?
You need higher quality of equipment and larger "chambers" to deal with low concentrations. Besides, in order to meet needs of plant (I mean, each plant has production output) you will need stated amount of raw materials, which you can find in material balance sheet of the applied technological processes. So, when using low-concentrated things as raw materials, even if yield is higher (it's higher usually by less than 10%, not really much) you will need larger volume of raw materials (since needed amount is usually given in mass of pure $SO_2$ in entering gas).