This won't be the case. Heat loss will be much, much higher from the
surface of the porridge than from the bowl. In fact, most of the heat
loss will be evaporative, at least during the first few minutes. As
such, the largest bowl will cool fastest, despite the smaller
surface:volume ratio.

But wouldn't the evaporative losses still be related to x squared, whilst
the available energy is x cubed? It's just an alternative form of surface
loss.

Christian.