I have done a resize on my Windows VM to increase the disk size, but in the VM console it does not show that it has resized; even though the OnApp UI does.
Windows Virtual Machine
Inside of the vm, Go to go to the Disk Management section
You should see your primary partition and an unallocated partition as large as the increase made to the disk.
Right click on the primary partition and run the extend option. Once this is done the two partitions will be combined to the expected size.
Note - Before making any disk related changes we recommend making a backup as a precaution.
As of yet, OnApp cannot extend a windows partition as a part of the resize disk task.