Are you sure about that? 150M documents in 50MB would be 0.3 bytes per document. 
Did you mean 50 GB?
JSON is inherently less space-efficient than statically-typed columns. The flexible schema implies that every document has to describe the names of its properties. Single-document compression doesn’t help that much because a property name usually only occurs once per document.
Is using 150GB of space causing problems for you? That’s only a few dollars of storage, at current prices…