In the context of SERVER-61281, memoisation was added for the computation of the approximate size of a document (code change). The approximate size of a document is computed as the sum of the bson object, some metadata and the cache size (code). With memoisation we do not compute the size again if the variable _snapshottedSize has already a value (code). However, as we load more of the document in the cache the size changes and the _snapshottedSize has not the correct size.

However, this seems to be by design (unit test). The invariant that checks whether the memUsage that we want to subtract is at least as much as the total _memUsage was added later in SERVER-72328.

So, we will remove the invariant.