it should be quite fast to serialize & de-serialize. not sure what programming language you are using.
the buffer will be larger as in bson the field names and data types for each field are in the encoding. if you look at bsonspec.org
you can see the encoding spec and from that get a feel for how large the documents will be.
700MB of data is fine but note that should consist of many bson documents (which could be appended all together in a single file) -- many of the bson encoders (depends on language) are non-streaming and assume a bson document fits in ram. In MongoDB there is a 16MB limit per document but that is database-related and not a bson limit per se.