i think you are confusing two different standards.
there is "micro-ATX" and "mini-ITX"
mitx is the smallest but has a few limiting factors such as only being able to support 1 graphics card, less fans and drives. also to my knowledge amd doesnt have a decent mitx board for their fx cpus.
matx is bigger so will require a bigger case but has more options and is amd friendly.
it is certainly possible to build a micro sized pc with full power! check out the image below
i recently specced a pc for someone (not the photo above, just similar) who uses an intel i5, 8gb ram and a gtx970 inside the 250d mini-itx case and it performs great (same as any larger pc). be prepared to shell out a bit more cost for mitx components but sure.. its 100% possible and works fine.
matx is not as small, runs a bit cheaper and supports more options though you will be looking at a mid sized case which may be larger than what you wanted.
generally mitx can use sff or mini cases while matx needs mid tower (though some mini towers do fit it)