The first difference you’ll notice between Mini ITX and ATX/MATX motherboards is the form factor. Mini ITX dimensions are as follows: 6.7” x 6.7”. Most Micro ATX boards measure 9.6” x 9.6”, and standard ATX motherboards measure 11.9” x 9.6”. Though companies play it pretty fast and loose with Micro ATX dimensions and shaping, Mini ITX boards are only ever one size and shape.
As they’re so small, Mini ITX motherboards don’t have anywhere near the number of facilities ATX boards do. For example, standard ATX boards can have as many as seven expansion slots, with Micro ATX boards holding up to four. Mini ITX boards only ever have a single expansion slot, and max out at two sticks of RAM with a 32GB cap.
Mini ITX motherboards are preferred by many for their power efficiency, portability, and low running volume. They can be used for PC gaming, but large, high-end graphics cards can rarely fit in a Mini ITX case, so if you’re looking to build a gaming powerhouse, you’re best off sticking with ATX.