sun1> df -k
Filesystem           1024-blocks        Used   Available Capacity  Mounted on
/dev/md/dsk/d1          10325760     2128111     8094392    21%    /
/proc                          0           0           0     0%    /proc
fd                             0           0           0     0%    /dev/fd
mnttab                         0           0           0     0%    /etc/mnttab
swap                     3258288          16     3258272     1%    /var/run
/dev/md/dsk/d3            494235        2397      442415     1%    /tmp
/dev/md/dsk/d4           3458922     2360622     1063711    69%    /users


sun1>  df -k | awk 'NR != 1 { print $0 }' | sort -r +n +3
/dev/md/dsk/d1          10325760     2128111     8094392    21%    /
/dev/md/dsk/d4           3458922     2360622     1063711    69%    /users
/dev/md/dsk/d3            494235        2397      442415     1%    /tmp
/proc                          0           0           0     0%    /proc
fd                             0           0           0     0%    /dev/fd

-r=Reverse
+n=Treat as numeric
+3=4th column