That includes computers the size of a quarter, like you see on the far right. The largest component on that computer is the mini-USB port.
(left to right) Pine64 A64+, Arduino Mega, Raspberry Pi 2, Arduino Uno, Raspberry Pi Zero, Arduino Micro, Adafruit Trinket, Arduino Mini, Adafruit Trinket Mini
These computers exist to perform specific purposes, like controlling a sensor or small array of sensors, or controlling LED's.
ASIC's, or Application specific integrated circuits are another form of computer that I use.
These ASIC's were calculating, or mining, a cryptocurrency called Bitcoin.
These graphics cards, or video cards, were also mining cryptocurrency, except one called Ethereum.
That is 2 thousand dollars worth of RAM next to a 2 thousand dollar CPU. I built this computer for virtualization environments. Essentially your computer runs one OS, often that single OS uses much of its resources. This computer was built to run up to hundreds at once.
Electromyographic prosthetics use multiple types of small computers, or microcontrollers. Here's my desk during the development process of a below-elbow 3D printed prosthetic with approximately 40 programmable gestures.
At its most basic level, this is a computer. It calculates Fibonacci numbers.
Here's a very simple computer that I soldered together, next to another one I was yet to begin. I could have taped the pin connections down straight before soldering, but they're not for shields/hats, rather individual wires.
So, from the size of a quarter, up to 6ft racks. Any computer, any problem.