I've seen cases where a newly extended VMFS datastore fluctuates between the old size and the new size. Virtual Machines running on th newly extended datastore prevents the correct size of the datastore from displaying.

I found the solution to the problem in KB Article 1002558

Products: VMware ESX and ESXi

The solution is:

1. Shut down or VMotion virtual machines running on the extended datastore to the ESX host that created the extent.
2. On all other ESX hosts other than the one that created the datastore, run vmkfstools -V to re-read the volume information.
3. Power on or VMotion the Virtual Machines back to the original ESX hosts.

This is not only on VI3 hosts but on:

VMware ESX 2.0.x
VMware ESX 2.1.x
VMware ESX 2.5.x
VMware ESX 3.0.x
VMware ESX 3.5.x
VMware ESXi 3.5.x Embedded
VMware ESXi 3.5.x Installable