www.ubergizmo.com/what-is/sim-card/

Subscriber Identity Module, or SIM, cards are used to act as a unique identifier for a mobile device on a wireless cellular network. They come in multiple size and shapes, but SIM, Mini-SIM and Nano SIM are really the same chip and connectors embedded ...

support.vodafone.com.au/articles/FAQ/What-different-types-of...

There are a few different SIM cards available. They all do the same job, they're just different sizes. Your device is designed to take one particular sized SIM, so make sure you're using the correct one.