Try something like this to replicate what the built-in formatters are doing:
PS C:\> Get-NaVol vol* | Get-NaSnapshot -SnapName nightly.* | select TargetName,Name,Created,Total,CumulativeTotal,Dependency | ft @{Expression={$_.TargetName};Label="Volume";Width=20},@{Expression={$_.Name};Label="Name";Width=25},@{Expression={$_.Created.ToShortDateString()};Label="Created";Width=12},@{Expression={ConvertTo-FormattedNumber $_.Total DataSize "0.0"};Label="Total";Width=10},@{Expression={ConvertTo-FormattedNumber $_.CumulativeTotal DataSize "0.0"};Label="Cumulative";Width=10},@{Expression={$_.Dependency};Label="Dependency"}
Volume Name Created Total Cumulative Dependency
------ ---- ------- ----- ---------- ----------
vol0 nightly.0 3/14/2012 20.6 MB 31.4 MB
vol0 nightly.1 3/13/2012 21.5 MB 75.3 MB
vol0 nightly.2 3/11/2012 20.4 MB 166.3 MB
vol1 nightly.0 3/14/2012 88.0 KB 172.0 KB
vol1 nightly.1 3/13/2012 88.0 KB 348.0 KB
vol1 nightly.2 3/11/2012 88.0 KB 704.0 KB
vol1clone nightly.0 3/14/2012 112.0 KB 200.0 KB
vol1clone nightly.1 3/13/2012 104.0 KB 408.0 KB
vol1clone nightly.2 3/11/2012 88.0 KB 800.0 KB
vol3 nightly.0 3/14/2012 52.0 KB 108.0 KB
vol3 nightly.1 3/13/2012 60.0 KB 228.0 KB
vol3 nightly.2 3/11/2012 60.0 KB 460.0 KB