According to Demand and Supply model（供给需求模型）:
To the supply aspect, the marginal cost is decreasing as the amount of commodity they produce is increaing(生产者：边际成本递减). That implies the more ml of coffee they produce, the less average cost they will have to pay for the addtional ml of coffee. Because the total cost of produce a cup of coffee include fixed cost(beginning investment, rent, machines, and here we can include the cup) and flexible cost(beans, electricity and heat).
cost of coffee per ml = fixed cost of coffee per ml + flexible cost per ml
As fixed cost of produce a cup of coffee remains the same or increase slightly (maybe the larger cup has to use a little more plastic), the more mls of coffee in the cup, the less average cost of each ml of coffee(that is the cost of coffee per ml). Moreover, the flexible cost of coffee per ml usually is extremely low compared to the fixed cost per ml.
To the demand aspect, the marginal utility of consumer is also decreasing as the amount of comodity they achieve is increasing(消费者：边际效用递减). That implies the more ml of coffee they have, the less money they're willing to pay for the additional ml of coffee. According to this theory, more mls of coffee in one cup, the less the average utility of each ml of coffee to the consumer - therefore less average value per ml to them. As the demand of the additional ml of coffee decrease, the balance price of supply and demand per ml will also decrease. As long as the supplier want to meet the balance point in the Supply and Demand model, they have to sell the additional mls of coffee cheaper.
The above anysis also applies to questions that is not in terms of coffee.(其他案例：同理)
关于供求模型我只能回答这么多，至于机会成本oppotunity cost和外部效应externality我不知道怎样应用，请其他回答者帮忙看看吧。
以上。
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