Why does a Lower Concentration of SO2 call for an increase of equipment dimensions in a Contact Process Plant? I'm working on a presentation based on an article about the Contact Process (production of sulfuric acid), and it says that using a low concentration of SO2 at the entrance of the converter (in which the oxidation of SO2 to SO3 takes place as an equilibrium reaction) is better in terms of conversion, but disadvantageous in that it would require using bigger plant equipment dimensions. 

Why does using a lower concentration of SO2 require a higher volume of such equipment? 

Wouldn't using more gas require more size?
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).