to think that Mrs Bear...

[DutchOma]

...spoiled her family by cooking three lots of porridge?

To clear up the surface area/volume line of thinking:

As the bowls increase in size, assuming a uniform shape, the surface area does increase.
However, the volume of porridge also increases, and increases at a greater rate.

For example:
In the small bowl, 100ml of porridge has a surface area of 100cm.
In the medium bowl, 400ml of porridge has a surface area of 200cm
In the large bowl, 900ml of porridge has a surface area of 300cm

So you can see (I hope) that the larger the bowl, the slower it loses heat, because while the surface area does increase, it doesn't increase enough to offset the increase in volume.