Mini-ITX (or just ITX, for short) is smaller than Micro- ATX.
ETA: NUC's aren't generally upgradeable, other than storage and memory. I'm not aware of any boards in that form factor that would allow you to drop in your own processor.
ETA 2: There's lots of ITX cases, from ones that can fit full size, 330mm GPUs to ones that can only fit the motherboard, low-profile CPU cooler, and an SFX PSU.