The missing memory is probably gone to serve as cache. You can use `free` to check this (looking at the last column):

    $ free
                 total       used       free     shared    buffers     cached
    Mem:      16331924   12547216    3784708          0     724568    6654748
    -/+ buffers/cache:    5167900   11164024
    Swap:      6291452       6944    6284508