Most things I cook are in flat rectangular trays.

There are two issues here.

Firstly, what matters is not the shape of the tray, but the shape and size of the individual pieces of food (you can put round meatballs into a flat tray, but they still remain round meatballs).

But the real key issue is not the shape, but the size. Shape will have an effect on the evenness of the heat (the corners of a rectangular piece of food will heat more than the flat sides). Whatever the shape, so long as the shape remains the same, the surface area will increase (or decrease) in proportion to the square of any one dimension, and that is the key issue. So, if you half the dimension, you will reduce by a quarter the surface area, so if 12mm (6mm from each side, as there will be two opposite sides) represents 50% of the depth of the item, then the surface area will have been reduced to a quarter of what it was, but the overall power only reduced by 37%, so the average power density will still increased 47% (although, as I said, if one is dealing with more realistic shapes, then this only represents average power distribution, and the actual power distribution will be very much more uneven).