TechRadar outlines the differences between different motherboard sizes and their different features and functionality ...

Mini ATX, FlexATX and Extended ATX (see PC motherboards). THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction requires permission.

As can be evidenced from the motherboard size chart above, there's quite a dramatic difference in scale between Mini-ITX and MicroATX, with more of a subtle difference when comparing ATX and EATX ...