Time Machine thinks back up is 3X the size of my HD.

I am trying to back up my MacBook Pro to my NAS with Time Machine. I have had this exact same set up working before, but I recently had a disc failure and am having to reset up my backups (long story). But I keep getting told the back up drive is not large enough.

My MacBook Pro has a 500GB drive in it with only 380GB used. And TIme Machine recognizes my NAS has the right amount of storage available at 780GB. But it says that my 1.03TB backup is too large for my available 780GB.

Any and all help is appreciated.

Exact Message:

"This backup is too large for the back up disk. The backup requires 1.03TB but only 771.01GB are available."