Fukishima containment.[1] The top of the containment vessel is shown in yellow, just above the red cylinder containing the reactor. The containment vessel was a heavy shell, but not much larger than the reactor. It had to contain any steam overpressure resulting from an accident, and didn't have enough volume that the steam pressure would decrease, and maybe condense. The surrounding building wasn't a pressure vessel and couldn't contain anything. Building panels blew out, leaving visible holes in the walls.

Three Mile Island containment.[2] The entire huge concrete and steel building around the reactor and support equipment is the containment vessel. When the reactor failed, radioactive steam escaped into the large containment vessel, where it was contained.

[1] https://world-nuclear.org/information-library/appendices/fuk...

[2] https://www.ans.org/news/article-3916/the-three-mile-island-...