The ZFS ARC cache tries to keep a balance between most frequently and least recently used data. Also, by default, ZFS ARC only fills out to a maximum of about half the available RAM. You can change that at runtime (by writing the size in bytes to

  /sys/module/zfs/parameters/zfs_arc_max

or setting the module e.g. in

  /etc/modporbe.d/zfs.conf

to something like this

  options zfs zfs_arc_max=<size in bytes>

). But be careful, as the ZFS ARC does not play that nice with the OOM killer.