The OS only takes up ~500MB. What you are seeing is the result of memory manufacturers reporting the amount of memory in decimal (base 10) while a computer can only read in binary (base 2). So 16GB to a computer looks like ~14.2GB of avaliable memory. You didn't lose anything and it's been this way since forever. Next time you get a new flash memory card, HDD, etc. look at what your computer says is free space and it will not be what is written on the box.
Originally Posted by CLZ
A 1GB is not actually 1,000MB as reported by manufactuers, it's 2^10 or 1,024MB. The OS is reporting the true value.