system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How are more than a hundred million transistors Fitted on an area of about a millimetre Square?
gpt
By making them *very* small.

The "Node size" chip manufacturers talk about is based on a measurement of how 'big' the transistors are (the exact meaning of this measurement varies between manufacturers). Usually this is measured in nanometres, i.e. one billionth of a metre. 

-"Hey! That would mean we need one metre to fit one billion" you might think; but consider they're not on a line, but on an area.

EIDT: To add, the manufacturing process involves shining light onto the material, and having the material brake down from the light. This is how you can make billions of nanometre-sized things cost-effectively. It's called "photolithography"