Maybe I missed it but I don't recall seeing a respected leaker indicate how much smaller the chassis would actually be. I had thought most of this was just from general consensus, because we already know the M1 Mac mini is mostly just empty space internally. I could be wrong though.
There are the Prosser renders, which look to be the same footprint, but a shorter height...
They also do not have a real proper airflow thought out, so more "pretty render" than functional design...
If you look at the 2018 Mac mini teardown, then the M1 Mac mini teardown, and then the M1 Pro / Max MacBook Pro teardowns...
The Intel mini is pretty full, the M1 mini is not; but the M1 Pro/Max in the MBPs is larger, so the room within the current mini chassis would seem appropriate for the higher powered SoCs...
So, even if it does get smaller, it's a matter of degree. I don't see it going to Apple TV size, and the truth of the matter is that the Mac mini isn't actually that small, once you compare the actual dimensions to existing Apple Silicon products. The thing is 19.7 x 19.7 x 3.6 cm or about 1.4 L. Even if you shave some of that off because of the corners and part of the base, that ends up being about 1.3 L.
Over on a Small Form Factor forum I frequent, we have an ongoing no-prize competition, Performance Per Liter...
Chassis volume is determined by a bounding box, no "shrinkwrapping", so the rounded corners & spaces around the "bottom circle / access hatch" would not be subtracted from the overall volume calculation...
IOW, a new Mac mini could easily lose one-third of the internal volume compared to the current M1 model and still house the M1 Max no problem.
I really don't think so, again, look at the assorted teardown videos...
Which is why I think there might be two Mac mini chassis designs going forward; the smaller / thinner model for the Mn-series SoCs, and the current model for the Mn Pro/Max-series SoCs...