For reference: an Intel Atom (e.g. Braswell) CPU has a TDP of 6W and a pretty large passive heatsink.
also for reference ~10W is Pentium ~100MHz territory. Afair 133MHz was the first one that required fan on top of the heatsink.
Fwiw my 486dx2-66 had a fan, and when that fan died we noticed because of corrupted memory crashing the system. Compared to modern CPU coolers it was downright tiny, but required.
From memory though, they were closer to 20mm or 30mm square though, right? (At least the outside package which I'm assuming is what the 12x12mm spec is referring to)