You're reading memory values wrong.
Ever since Mavericks, RAM compression and caching have been introduced. So free RAM is wasted RAM. What you should look at is the memory pressure graph. If it's green, all well and good. Free RAM values are irrelevant, as OS X will use as much RAM as it can for compression and caching.
With these two, 8GB of RAM will act like 12GB of RAM, or around that. I'll repeat myself once more - free RAM is wasted RAM.