First they can sell the smaller cans for more money for the same amount, which doesn't matter from cost of making it, but does matter to the amount someone will consume.
It's the same rationale behind 99% payouts of slot machines in the casino. It just allows someone to play longer.
Soda is loaded with sugar and nothing else. It has to be extreme moderation to not have an effect on someone's health (relative to what we think of as normal now). Coke is trying to reframe soda as somehow ok.
One way to do that is to have small cans and say 'no problem they cans are obviously tiny'. Someone will buy them and drink as much as they want anyway because what they want isn't to take a long term view of their health, they want a rationalization. That is what Coke is offering and it does happen to be what people want.