As i can see It looks very similar, but it might have differences.
Sophia is a document storage.
Where key-value treated as a special case of document storage.
It is not a columnar storage, yet the scheme definition is very likely in some sense.
It is more closer to a row storage.
Basically, Sophia storage format allows to define field types in a very likely way
how WiredTiger does. Fields are part of document. Probably a more close comparison would be
to imagine a JSON document. It has named fields, but we only save values of those
(because we know the scheme and their types).
Yet, fields does not stored in the same order they are defined. Fixed sized fields (integers) are
grouped to save space, etc.
One of the the nice features of the new scheme if ability to store duplicate fields only once.
For example, by using the default scheme:
db.name.scheme.key = u32,key
db.name.scheme.value = string
We can save space by storing value field copies only once per range.
This is beside a common compression (lz4, etc).