Of possible interest:
ZFS Space Accounting (Solaris ZFS Administration Guide)I don't know how diskutil (for Disk Utility) calculates the total size, but I guess that partition type zfs_filesystem_proxy is a factor.
For reference only, the example below is for one of my physical disks where there are both parent and child file systems.
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sh-3.2$ date
Wed 19 Sep 2012 19:33:33 BST
sh-3.2$ diskutil list disk8
/dev/disk8
#: TYPE NAME SIZE IDENTIFIER
0: zfs_pool_proxy zhandy *635.7 GB disk8
1: zfs_filesystem_proxy Pocket Time Machine 113.7 GB disk8s1
sh-3.2$ zfs list | grep zhandy
zhandy 527Gi 55.9Gi 407Gi /Volumes/zhandy
zhandy/Pocket Time Machine 49.9Gi 55.9Gi 49.9Gi /Volumes/zhandy/Pocket Time Machine
sh-3.2$ diskutil info disk8
Device Identifier: disk8
Device Node: /dev/disk8
Part of Whole: disk8
Device / Media Name: zhandy
Volume Name: zhandy
Escaped with Unicode: zhandy
Mounted: Yes
Mount Point: /Volumes/zhandy
Escaped with Unicode: /Volumes/zhandy
File System Personality: ZFS
Type (Bundle): zfs
Name (User Visible): ZFS
Content (IOContent): zfs_pool_proxy
OS Can Be Installed: No
Media Type: Generic
Protocol: USB
SMART Status: Not Supported
Volume UUID: 0DE88D2B-F43B-5BCB-B91A-4E7C73218D00
Total Size: 635.7 GB (635655159808 Bytes) (exactly 1241513984 512-Byte-Blocks)
Volume Free Space: 60.1 GB (60058008064 Bytes) (exactly 117300797 512-Byte-Blocks)
Device Block Size: 512 Bytes
Read-Only Media: No
Read-Only Volume: No
Ejectable: Yes
Whole: Yes
Internal: No
OS 9 Drivers: No
Low Level Format: Not supported
sh-3.2$ diskutil info disk8s1
Device Identifier: disk8s1
Device Node: /dev/disk8s1
Part of Whole: disk8
Device / Media Name: zhandy/Pocket Time Machine
Volume Name: Pocket Time Machine
Escaped with Unicode: Pocket%FF%FE%20%00Time%FF%FE%20%00Machine
Mounted: Yes
Mount Point: /Volumes/zhandy/Pocket Time Machine
Escaped with Unicode: /Volumes/zhandy/Pocket%FF%FE%20%00Time%FF%FE%20%00Machine
File System Personality: ZFS
Type (Bundle): zfs
Name (User Visible): ZFS
Partition Type: zfs_filesystem_proxy
OS Can Be Installed: No
Media Type: Generic
Protocol: USB
SMART Status: Not Supported
Volume UUID: 764AEB8A-F9C5-5442-9058-B7B4F02361E7
Total Size: 113.7 GB (113663110144 Bytes) (exactly 221998262 512-Byte-Blocks)
Volume Free Space: 60.1 GB (60058008064 Bytes) (exactly 117300797 512-Byte-Blocks)
Device Block Size: 512 Bytes
Read-Only Media: No
Read-Only Volume: No
Ejectable: Yes
Whole: No
Internal: No
sh-3.2$ zfs get all "zhandy/Pocket Time Machine"
NAME PROPERTY VALUE SOURCE
zhandy/Pocket Time Machine type filesystem -
zhandy/Pocket Time Machine creation Fri Jun 22 15:51 2012 -
zhandy/Pocket Time Machine used 49.9Gi -
zhandy/Pocket Time Machine available 55.9Gi -
zhandy/Pocket Time Machine referenced 49.9Gi -
zhandy/Pocket Time Machine compressratio 1.12x -
zhandy/Pocket Time Machine mounted yes -
zhandy/Pocket Time Machine quota none default
zhandy/Pocket Time Machine reservation none default
zhandy/Pocket Time Machine recordsize 128Ki default
zhandy/Pocket Time Machine mountpoint /Volumes/zhandy/Pocket Time Machine default
zhandy/Pocket Time Machine checksum on default
zhandy/Pocket Time Machine atime off inherited from zhandy
zhandy/Pocket Time Machine devices on default
zhandy/Pocket Time Machine exec on default
zhandy/Pocket Time Machine setuid on default
zhandy/Pocket Time Machine readonly off default
zhandy/Pocket Time Machine snapdir visible inherited from zhandy
zhandy/Pocket Time Machine canmount on default
zhandy/Pocket Time Machine copies 1 inherited from zhandy
zhandy/Pocket Time Machine version 5 -
zhandy/Pocket Time Machine utf8only off -
zhandy/Pocket Time Machine normalization none -
zhandy/Pocket Time Machine casesensitivity insensitive -
zhandy/Pocket Time Machine refquota none default
zhandy/Pocket Time Machine refreservation none default
zhandy/Pocket Time Machine primarycache all default
zhandy/Pocket Time Machine secondarycache all default
zhandy/Pocket Time Machine usedbysnapshots 325Ki -
zhandy/Pocket Time Machine usedbydataset 49.9Gi -
zhandy/Pocket Time Machine usedbychildren 0 -
zhandy/Pocket Time Machine usedbyrefreservation 0 -
zhandy/Pocket Time Machine logbias latency default
zhandy/Pocket Time Machine sync standard default
Broader views of all currently used ZFS file systems, none of which has a quota:
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sh-3.2$ date
Wed 19 Sep 2012 20:38:39 BST
sh-3.2$ df -h -T zfs
Filesystem Size Used Avail Capacity iused ifree %iused Mounted on
/dev/disk3 515Gi 292Gi 224Gi 57% 4277743 468875568 1% /Volumes/gjp22
/dev/disk8 463Gi 407Gi 56Gi 88% 109423 117300305 0% /Volumes/zhandy
/dev/disk8s1 106Gi 50Gi 56Gi 48% 2791 117300305 0% /Volumes/zhandy/Pocket Time Machine
/dev/disk10 484Gi 440Gi 44Gi 91% 6900 91431056 0% /Volumes/tall
/dev/disk10s1 54Gi 11Gi 44Gi 20% 2041 91431056 0% /Volumes/tall/backups
/dev/disk10s1s1 44Gi 824Ki 44Gi 1% 143 91431056 0% /Volumes/tall/backups/LaCie d2 Extreme
/dev/disk10s1s1s1 44Gi 688Ki 44Gi 1% 126 91431056 0% /Volumes/tall/backups/LaCie d2 Extreme/11G
/dev/disk10s1s1s2 44Gi 516Ki 44Gi 1% 78 91431056 0% /Volumes/tall/backups/LaCie d2 Extreme/12A
/dev/disk10s1s2 47Gi 3.3Gi 44Gi 8% 3027 91431056 0% /Volumes/tall/backups/blocky
/dev/disk10s1s3 336Gi 292Gi 44Gi 88% 4277884 91431056 4% /Volumes/tall/backups/gjp22
/dev/disk10s1s4 437Gi 393Gi 44Gi 91% 109386 91431056 0% /Volumes/tall/backups/zhandy
/dev/disk10s1s4s1 96Gi 52Gi 44Gi 55% 2781 91431056 0% /Volumes/tall/backups/zhandy/Pocket Time Machine
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sh-3.2$ date
Wed 19 Sep 2012 20:39:00 BST
sh-3.2$ zfs list -o name,used,available,referenced
NAME USED AVAIL REFER
gjp22 339Gi 224Gi 292Gi
tall 1.75Ti 43.6Gi 440Gi
tall/backups 872Gi 43.6Gi 10.9Gi
tall/backups/LaCie d2 Extreme 1.98Mi 43.6Gi 824Ki
tall/backups/LaCie d2 Extreme/11G 688Ki 43.6Gi 688Ki
tall/backups/LaCie d2 Extreme/12A 516Ki 43.6Gi 516Ki
tall/backups/blocky 3.31Gi 43.6Gi 3.31Gi
tall/backups/gjp22 348Gi 43.6Gi 292Gi
tall/backups/zhandy 510Gi 43.6Gi 393Gi
tall/backups/zhandy/Pocket Time Machine 52.5Gi 43.6Gi 52.5Gi
zhandy 527Gi 55.9Gi 407Gi
zhandy/Pocket Time Machine 49.9Gi 55.9Gi 49.9Gi
VisuallyThe traditional design of Disk Utility does not lend itself to modern approaches such as Apple CoreStorage or ZFS.
In the first screenshot below, three of four items that appear within a CoreStorage logical volume group are truly not within that group. In the second and third shots, some (not all) child file systems appear to be peers of their parents; and so on.

- things not within an LVG appear to be within that group
- 2012-09-19 20-48-23 screenshot.png (164.32 KiB) Viewed 39 times

- a child appears at the same level as its parent
- 2012-09-19 20-43-48 screenshot.png (203.09 KiB) Viewed 39 times

- some child file systems appear nested
- 2012-09-19 20-43-59 screenshot.png (199.66 KiB) Viewed 39 times
All things consideredI shouldn't expect diskutil or Disk Utility to wholly reflect the pool/volume management capabilities of ZEVO.