Firstly, check where your swap file is (it will be a ZFS volume created during the Solaris installation):
bash-3.00# swap -l
swapfile dev swaplo blocks free
/dev/zvol/dsk/rpool/swap 256,1 16 4194288 4194288
Then you will need to unmount it:
bash-3.00# swap -d /dev/zvol/dsk/rpool/swap
You should validate it is unmounted:
bash-3.00# swap -l
No swap devices configured
Then you can resize the ZFS volume (just give it the pool name and the volume name):
bash-3.00# zfs set volsize=16G rpool/swap
And then add it back into your swap space:
bash-3.00# swap -a /dev/zvol/dsk/rpool/swap
And now we see the swap space is back online and larger than before:
bash-3.00# swap -l
swapfile dev swaplo blocks free
/dev/zvol/dsk/rpool/swap 256,1 16 33554416 33554416
Enjoy !!