Hi,
When the opposite would make more sense to me, i.e. with more area , more surface to radiate the heat away, hence more current per cm2 should be allowed
This is correct: more area --> more power to dissipate
But the ratio will be less.
Imagine:
An area of 1cm x 1cm ... let´s imagine it can dissipate 1W...
Now use 9 identical such areas in an 3 x 3 arrangement. Best if you focus on the center area, it is surrounded by 8 areas, that also generate heat.
A single is surrounded by "cold" area, where in the 3x3 arrangement the center one is surrounded by "hot" area.
***
Now you may say: but the "cold" border of the 3x3 arrangement is longer than with the 1cm x 1cm area.
True. But the length of the border increases with the square root of the area, where the power increases linearely with the area.
border length of a 1x1 cm square = 4cm, area = 1 cm^2
border length of a 3x3 cm square = 12cm, area = 9 cm^2. ( border length is
not 9x 4cm = 36 cm)
****
As FvM mentioned, you must not only focus on the vertically radiated heat, but also on the horizontally spread heat.
Klaus