Other posts have looked at how vendors will often aggressively discount larger sizes of highly perishable discretionary items – things like popcorn and soda at movie theatres, slushies, etc.
There are a couple of things going on here. The first is that the vendor recognizes the diminishing marginal utility of their product (that each additional sip of slushy provides a little less enjoyment than the one before it) and price accordingly. They know you value the extra in the medium large sizes less so they discount them heavily.
The other factor is that those products are highly perishable. Buttered popcorn is only warm and delicious for a short time, it quickly becomes a disgusting congealed mess. Same things with soda – ice-cold and sparkling is soon warm and flat.
That means that you can't stockpile the product. If you could the large discounts would cannibalize future full price sales – you would simply save half your slushy for the next hot day. Because that is not an option the vendor is happy to offer the discount to encourage you to buy more.
Stockpiling might be out but sharing or splitting is easy. Two small popcorns would cost $5.69 each, splitting one medium (the same amount) costs just a little over half that, about $3.10. Same thing with soda.
It's obvious, but often not taken advantage of because there is also a hurdle effect involved. Two guys going to see the Expendables might feel weird about sharing snacks. Fair enough, but that is a decision to not take advantage of the potential savings that are available.