In the last months I cooked a lot of jelly and found an interesting pattern from my ceran stove top: When the heating unit barely fits under the pot the juice will boil over. However if I use the smaller circle (same place at the stove, same pot), it boils but does not overboil. Why does the diameter of my cooking field have an effect on the boil-over-behavior of my juice?
One is heating the to edge of the base and consequently also the pan side-wall, the other is not.
The cooler pan side-wall will just make the difference between it boiling over and not.
This is assuming the centre of the pan is being heated to exactly the same degree on each, which is a larger variable.