Some times I do not think the purchasing power is the reason why the rich buys commodities in large quantities while the poor buys in small quantities, yes the rich may have the money more than the poor but the poor sees buy things in buck as a waste of money because they are used to buying things in small quantities but if you check the amount they rich and the poor spent on a particular item you will realize that the poor even spends More without knowing.
The poor lacks the wisdom to calculate very well that's why you see them spending more than the rich when they don't have.
From this perspective there is no difference because the money the rich will spend in buying the goods ones to last for two months is exactly the amount the poor might spend getting it everyday for two months, so actually the one time purchase is what makes a little different about it, although the consuming method might differ a lot from that of the poor because a poor person takes there food items slowly knowing that it cost them a lot to get but the rich doesn't regard that and they are ready to use it even when is not necessarily for them, so for economical part of it the poor does better.
There is a big difference between a person who buys goods in large quantity and a person who buys in small numbers, for example a person can spend $50 in getting a carton of biscuits spends less than a person who buys the biscuits one daily. The consuming method of the poor is faster than that of the rich you will agree to this because the poor eats more than the rich.