system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the logic behind fast food places making their next size up in soft drinks only like 25 cents more. Wouldn't they make more money if the difference in price was proportional to the difference in size?
gpt
I believe there's two factors at play:

1. For something like a soda, it's mostly water, which is essentially free. The cost is mainly the container. The volume of a container increases as a cubic function, while the cost of the material is closer to a square, so bigger containers cost less per unit volume. Taken together, that 25 cents is mostly profit. 

2. Psychology. People sometimes ignore the base cost, since they either want a particular product, like a soda, or not. They are, however, more conscious of the upgrade cost. By setting the base price high, but then having small steps to the large size, the net effect is more people buy the most expensive size. In contrast, if the base price was low, but the steps were large, more people might buy the small size.