Yes, Mini ITX is the smallest form factor you can find for a consumer-grade x86 device. And SteamOS, while not officially available, can be run on pretty much any x86 PC as long as it’s fully AMD (CPU and GPU). There are some builds with support for Intel and Nvidia, but the overall experience is not as good.
But what people have to understand is that Mini ITX itself has a huge range of sizes in terms of the final build. The motherboard is standardized, but the cases are not. For example, I have one that I use as an HTPC, but it’s on the larger side at about 24L. To put that into perspective, the Steam Machine case is only 4L, while a standard ATX build (mid-tower) has around 45L.
And while you can definitely find ITX cases of that size, building in them is super tricky. Most are designed to use APUs exclusively, and the ones that support a discrete GPU have some very strict size limitations. So much so that for a build the size of the Steam Machine, you really can’t be that much more powerful. The vast majority of those cases only fit GPUs up to about 20 cm in length, which means you’re stuck with the RTX 4060 (which has two models meant for small ITX builds) or weaker cards. Even the closest GPU to the one in the Steam Machine, the AMD RX 7600, doesn’t have a model that would fit in most of those cases.
Of course, if you’re willing to go larger, you can fit a lot of different hardware and still technically have an “ITX build.” So honestly, I wouldn’t say the Steam Machine is ITX, it’s much closer to a mini-PC, which is non-standard but can go as low as 2L in some cases. The Atari VCS, for example, is around that 2L mark.
