When creating capped collections you have to define its maximal size in bytes which is size of uncompressed data.
However, since wiredTiger stores data compressed on disk, it is very difficult to guess the right "size" not to exceed a certain amount of data on disk because the compression factor (and thus the "storageSize") may vary.

Therefore, it would be much better to be able to define the maximal "storageSize" when creating a capped collection. For downwards compatibility, the "size" parameter can co-exists.

This feature is even more important as long as capped collections cannot be resized.