Postgres 9.4 has a new column format called "JSONB" that is a binary representation of JSON. The coolest thing about this new column type is that it supports indexes, so you can now query JSON columns much faster. However, a side benefit is that it is also slightly smaller. I recently did a test where I took a 53 MB table of mostly JSON data and converted it to JSONB. Before and after the conversion I did a
VACUUM FULL on the table. Space savings was about 12.3%.
Just one observation you might want to consider if you are thinking about switching formats.