I think the situation is this: there used to be a small, medium and large drink at McDonald's. The small is no longer commonly used. It was quite small, about 8 oz. The medium cup is still used, and although it is now the smallest cup, it retains its identity as a medium drink.

On a barely related topic, why is the word "medium" used to describe that which is in the middle
(as in "The play begins in media res with the ghost of King Hamlet stalking the walls of the castle"), as well as the means by which something is transmitted or communicated (as in "The artist was primarily a sculptor in the 1960's but now he has moved on to other media such as printmaking and painting")?

I use the "Black Mamba" brand of condoms myself
http://www.mwscomp.com/movies/mol/inlines/i-2_prot.jpg
In reply to:

Do other countries have "unit pricing?"

I don't know but mine's not for sale. ;]