Just been watching some Youtube on mini gaming pc's. some even clip on the back of your monitor. There are lots to look into.
Minis and laptops have the same kind of advantages (size, portability) and limitations (cooling and power mainly as a result of thier size) and tend to use the same components. The same may be said of all-in-one machines which are basically minis built in to a monitor BTW. As such you would expect a gaming mini to be some distance behing a full fat desktop machine in terms of both performance and VFM but similar to a gaming laptop with the same spec'.
You can easily make a mini-ITX build these days with equivalent performance to a full gaming PC and still be the size of a shoe-box or small backpack. It is when you get so small that normal graphics cards don't fit that you end up with more of a NUC or terminal-type machine that is mainly for internet, home-office or HTPC use.
Not a gaming pc, but i have a mini pc right next to me. It is from my mom's office, she brought it here when covid lockdown hit and has stayed here ever since. To say it is garbage is an understatement. My previous PC, which i bought in 2012, wipes the floor with this square of doom. It is infuriating to attempt to do almost anything in this thing (and i dont mean gaming, that pc would die if it even thought about it - i mean common office stuff), i wish i was there when my mom bought it because she really got badly scammed with it.
