PostgreSQL: Separate tables vs single table to perserve disk space?
Because the resulting combined table would have unused rows for records from each respective table, the likelihood is that you would not gain space, but lose it. 

More importantly than that, I would highly discourage making an architecture decision of that nature for reasons of disk-space. Disks are cheap--a well designed and functional application is not. If there is an issue of run-time performance (ie all 46 gb is being loaded into memory/swap space) then you may have an optimization and tuning issue on your hands.