Prices:
Small - $3 for 1150kJ
Medium $5.90 for 1740kJ
Large $6.30 for 2110kJ
So the cheapest per kJ/litre is small.
I am no mathematician but small is the cheapest, large is next, followed by medium.
Thus if you want a large quantity of Shake, buy multiple Smalls.
The above is for chocolate but other flavours, despite fluctuating in kJ, still show Small as the cheapest.
How often do you think people go to Maccas and order a small meal? Apparently for some it's the equivalent of buying small sized condoms.
For every one person buying the $3 shakes and $2 hamburgers, there are dozens ordering straight from the full priced menu, money be damned.