Troubleshooting zfs snapshot size usage

gistfile1.txt

$ sudo zfs list -r rpool -o name,used,compression,readonly,mounted
NAME                         USED  COMPRESS  RDONLY  MOUNTED
...
rpool/insig/tmp/playground  6.91M       lz4     off      yes

$ ls -lh /tmp/playground/
total 512
-rw-rw-r-- 1 rsyring rsyring 2 Apr 14 14:59 foo

# I think snapshot @5 should have more used space.
$ sudo zfs list -t snapshot -r rpool/insig/tmp/playground 
NAME                           USED  AVAIL  REFER  MOUNTPOINT
rpool/insig/tmp/playground@4     9K      -    19K  -
rpool/insig/tmp/playground@5     9K      -  3.45M  -
rpool/insig/tmp/playground@6  3.44M      -  6.89M  -

rsyring@loftex:/tmp$ ls -lh playground/.zfs/snapshot/4/
total 512
-rw-rw-r-- 1 rsyring rsyring 0 Apr 14 14:46 foo

# This 3.5M file doesn't seem to show up in the ZFS snapshot listing.
rsyring@loftex:/tmp$ ls -lh playground/.zfs/snapshot/5/
total 3.5M
-rw-rw-r-- 1 rsyring rsyring    0 Apr 14 14:46 foo
-rw-rw-r-- 1 rsyring rsyring 3.5M Apr 14 14:57 lxml-3.7.3-cp35-cp35m-linux_x86_64.whl


rsyring@loftex:/tmp$ ls -lh playground/.zfs/snapshot/6/
total 6.9M
-rw-rw-r-- 1 rsyring rsyring    2 Apr 14 14:59 foo
-rw-rw-r-- 1 rsyring rsyring 3.5M Apr 14 14:57 lxml-3.7.3-cp35-cp35m-linux_x86_64.whl
-rw-rw-r-- 1 rsyring rsyring 3.5M Apr 14 14:59 lxml-3.7.3-cp35-cp35m-linux_x86_64.whl.2