Comment by kens

The microcode was so huge that they had to use a semi-analog ROM that held two bits per transistor by using four transistor sizes.

As far as the layout, the outputs from the microcode ROM are the control signals that go to all parts of the chip, so it makes sense to give it a central location. There's not a lot of communication between the upper half of the chip (the bus interface to the 8086 and memory) and the lower half of the chip (the 80-bit datapath), so it doesn't get in the way too much. That said, I've been tracing out the chip and there is a surprising amount of wiring to move signals around. The wiring in the 8087 is optimized to be as dense as possible: things like running some parallel signals in silicon and some in polysilicon because the lines can get squeezed together just a bit more that way.