Why are GPU dies so much physically bigger than CPU dies?
GPUs are parallel processors.  Their performance scales linearly with die size, while bigger dies can be run at lower clocks and so be more efficient while still being faster.  Therefore, it makes sense to have the die as large as is economically feasible as it will be faster and more efficient.  

CPUs have much less parallelism, and so the return from adding more die area is much smaller, or for many applications, non-existent or even negative.  Therefore there is a relatively optimal die size for a consumer CPU on each node, and it is generally quite small.  The comparison to the comparison to the 175mm^2 9900k is actually slightly misleading, only about 100mm^2 is CPU.  The remainder is a relatively large GPU, video decoder, IO, etc.  Since there is also a minimum die size needed to fit all the IO pins, low power CPUs dies each generation are often mostly GPU to fill up the unused die space.