MongoDB’s WiredTiger storage engine compresses indexes and collections by default, so it is expected that the data size can be bigger than the storage size. The ratio between storage and data will depend on how compressible your data is as well as the compression algorithm used.
While I believe the behaviour you are describing is expected, I would note that MongoDB 4.2.1 was released almost two years ago (Oct 22, 2019). I recommend upgrading to the latest 4.2.x release (currently 4.2.23) as there have been many bug fixes and stability improvements since 4.2.1: 4.2 Changelog. Minor updates within the same release series do not introduce any backward breaking changes.