The Ivy Mike "Sausage" was big but it didn't fill the entire building - there was a lot of instrumentation for that test. There was actually a weaponized version of the Ivy Mike Sausage planned which was cancelled when they found out how to make "dry" bombs using lithium deuteride.

You can see the Sausage in the pics on the Wikipedia page:

https://en.wikipedia.org/wiki/Ivy_Mike

that's the point, was not a weaponized device, does not mean it did not go boom.