https://www.theguardian.com/technology/2013/dec/12/ps4-and-x...

> The PlayStation 2, meanwhile, had the original PlayStation chipset built in, so it ran pretty much any PSone title – and when that chip wasn't being used for backwards compatibility it doubled as an input/output processor, which was pretty canny.

Under heading “I/O”

Essentially the thicc PS2s include the PS1 chip and normally use it for IO and also for PS1 compatibility.

The original fat PS2 including majority of the PS1 hardware physically in the machine. More and more of it got offloaded to pure software emulation with later model revisions.

I, too, would like to see info on this supposed "spare die space on the USB controller" claim. Because it would be really neat if true lol