They have not been shrinking the minimum size since they did the RPi Zero. So approximately the size of a memory DIMM. The "full size (smaller then a paperback) version has been getting much much faster while sort of holding the lid down on the price (they added some variants with more RAM that cost more dollars, although tens of more dollars not $100s, so still a great deal; and the RPi 4 and RPi 5 have much faster I/O (the 4 added high speed USB, and 5 added I think a serial PCI…and maybe M.2 storage connectors or something that gets you past the SD requirement).
However the outgoing Lightining standard Apple had some cables like the lightning to HDMI that had an ARM Cortex M0+ that booted a stripped down Unix variant and runs an application that reads data from the lightning side and decodes MPEG or some other lossless motion encoded video. So while you can’t fit an RPi in a cable, you can get a whole CPU in there. The CPUs Apple use actually boot an image the phone provides them, I don’t remember if that was because they coulndn’t fit a big enough ROM in there or not, but I expect you can fit a masked ROM in very very little space. Apple was either looking to save money or they wanted to be able to fix bugs later.
I don’t know if they have done that with any non-Lighting cables. The USB-C HDMI cable very likely doesn’t need that, I think USB-C has an “alternative mode” to run HDMI over some of the pins (I know DVI does), and expect that is a lot easier to use then booting a whole other system and doing real time video compression…
