AMD Athlon XP T-Bred core, the same chip was used in low end CPU for around $60 and top-end CPU for around $500. And because yields were so good, you could basically buy almost any 60 buck chip and overclock it to the same clocks as $500 chip. Golden times of CPU overclocking. How about that?
Jokes aside, consoles use APUs - and just look at AMD Renoir, using the same APU chip from top to bottom with numerous configurations of CPU and GPU cores/clocks. Nothing new here.
Regarding the impact on motherboard and size of the console - because chip will use less compute units at lower frequency, it will consume significantly less power. Less power = less heat. This means much smaller box, simpler and cheaper cooling and PSU. Also, less memory chips and narrower bandwidth mean cheaper and simpler motherboard design.
The size of the APU chip itself never mattered that much, unless you try to build some kind of an ultra-slim, ultra-light laptop like macbook air, where even the die area could matter and the number/location of memory chips. For regular laptop and consoles - the main points driving the design always have been TDP of the chip and number of lanes, their efficiency needed in motherboard to connect the rest of the components. And both of these will be significantly lower and cheaper in XSS compared to XSX.