At least 1 disk in a protected VM going through initial sync has a size that is not a full GB (i.e. 100.0199 GB). In VCenter, this is typically not a problem. However, in Hyper-V, there are limitations with utilizing fractional GB provisioned disks. When SCVMM receives the API call from Zerto to create a disk of an exact size, it does not properly create the mirror of the exact same size, instead it rounds up to the nearest GB.
Due to this, the mirror will be of differing size from the associated protected volume and cause this alert found in the index.html of the log bundle.
This same limitation can cause similar issues when a fractional GB sized disk is created directly on a Hyper-V host rather than the SCVMM as the Hyper-V host will allow it. However, SCVMM will round up to the nearest GB in terms of reporting and therefore incorrectly report the size of the disk back to Zerto via reflection collection. Then, at VPG creation, the mirror will be made too large and see a similar behavior. For this reason, it is best to create all disks within SCVMM to avoid this issue.