How did you determine that? The two should use about the same amount of memory. The only diffence is that ZFS uses ARC and btrfs uses the page cache. ARC is not

How did you determine that? The two should use about the same amount of memory. The only diffence is that ZFS uses ARC and btrfs uses the page cache. ARC is not reported the same way as the page cache though, which might give the appearance of requiring more.