According to https://openzfsonosx.org/wiki/Performance the default ARC size of O3X is 25% of the physical RAM. It turned out to be false for 1.3.1-RC2. It was 100% - 1GB like FreeBSD. Also the OID is renamed from zfs.arc_max to kstat.zfs.darwin.tunable.zfs_arc_max . I took a liberty to add it to the wiki.
Is there any way to set this on boot? /etc/sysctl.conf does not work on Yosemite…
Dan the O3X Newbie, ZFS user since FreeBSD 7