Smallest case that actually fits gaming rated components such as a decent sized gpu and cpu is the Intel NUC, but that's an ordered/pre-built machine, not something you build yourself.
MITX cases generally come in 2x size differences, those @ 13L and below and those @ 13L and above. The difference there is pricing as the smaller cases are boutique cases, and easily sit in the $250+ range, and the larger cases are mass production, like from NZXT or Fractal Design etc. and are considerably cheaper.
MITX cases are backwards. ATX cases you'll generally have plenty of options, all your equipment will fit, rarely any consideration for sizing other than some cpu coolers. Not so with mITX, the smaller the case, the higher the research necessary to see what does and what does not fit, and whether that's a deal breaker.
For instance most of the smaller cases are limited to FE 2 to 2½ slot cards and may or may not accept anything longer than a 3080, plus the psu is of vital importance, there's very few ultra high power SFX or SFX-L psus and even then, some cases are restricted to SFX only.
MITX is all about compromise, either lowering expectations to fit the case, or changing the case to fit the expectations.