Virtual Memory size

Both of our Macs are working great with SL.

Had noticed in Activity Monitor on my MBP, it's been indicating VM size of 150 GB.
That's the entire sum of free space on that volume.

Picked up my wife's MB this morning, it's indicating VM size at 100 GB.
Meanwhile, it's an only an 80 GB drive with 50 GB of free space.

Could anyone explain what I'm really looking at?

The way I understand it is that the VM Size is the the total amount of VM that 'could' be used based on the processes that are currently running but does not reflect the VM actually in use at the time.

The 'real' VM amount can be calculated from the total size of the swap files found in /var/vm.

This is my best understanding anyway. Of course the VM size you see has gone up significantly in SL due to the amount of VM available to 64 bit processes now.