Also, you can find previous announcements of Hitachi's μ-Chip series. In 2001, evidently, they put out a 0.4 mm x 0.4 mm chip.

Not being programmable at all and just transmitting a 128 bit number would help get the size down.

Let's compare to a Monza R6 chip that was introduced in 2014, I think. This thing is 0.464 by 0.442 mm according to the datasheet, so quite a bit larger even than the 2001 read-only μ-Chip.

But it it has a two-way communication with the controller, and writable memory. You can enable password protection and such.

The newer M800 series is smaller: 0.247 mm × 0.362 mm, but still larger than the 2005 read-only μ-Chip. There are more features: fatter datasheet. Things like a privacy mode: tag remains radio silent unless it sees a specific 32 bit code from the reader.

You know how you can hold a totally unrelated RFID tag to a door reader and have the reader beep, indicating it has communicated with the tag? This looks like the feature that would prevent that. That could be useful.