because if you are aiming to breastfeed you hopefully will not need a box that would get wasted... where as hopefully a few oz from a small carton may just be enough to get you over a difficult patch one day, to give more than a few full feeds might not help your own milk supply...
... these companies are really clever, in that they are well aware that they cannot advertise the first stage milk, but know that how they advertise the follow on stuff affects the use of their first stage.... follow on and above is generally just an advertising ploy....