fstab / tmpfs mount issue

I have my tmpfs set to 8g (18gb memory on a VM)

Code:

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tmpfs                  /dev/shm                tmpfs  size=8g        0 0

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Whenever i reboot the VM the filesize of tmpfs is 6gb.

When i run "mount -o remount tmpfs" the file size changes to the 8gb it should be.

Anyone have a clue where centos is pulling the initial 6gb size from?

Thanks in advance