The hidden pitfall in this question is that the most obvious answers come with external power supplies. Some are half the size of the computer.
e.g. https://www.servethehome.com/lenovo-thinkcentre-m920x-tiny-r...
Some models can be powered over USB-C, allowing them to run off compact GaN chargers, monitors, etc.
https://www.amazon.com/MINISFORUM-7940HS-Radeon-USB3-2-PCIe4...