Random magma cube spawned in my tunnel (1 block wide, 2 heigh) and suffocated in the wall. I can't tell, whether it was a big one to begin with, but a medium at the very least.

So far this was a one time occurrance in my survival world. It was a fresh 1.16 world never played on a prior version.

Biome: It was a tunnel through nether rack, but I would say it was still in the vasinity of a Basalt biome