I believe you are talking about the capacity of the thin luns. You will need to make sure that both the volumes were claimed as thin and thin rebuild enabled. if either of these two parameters is not set. it will populate the entire capacity of the volume with zeros. This will happen to the side which is getting rebuilt.
Hi All,  We have a A+A setup across vplex. When we checked the distributed volumes configured on VPLEX, we found that no matter the secondary volume configured on which storage, all the secondary volume’s utilized as 100% even the primary volumes hardly reached 100%.  Is this common behavior or is it because of some parameters configured anywhere.? 
vplex secondary volume utilized at100%