Why does the replicated disk become larger than original

We have two physical (one is 2012 R2, the other is running Server 2016) servers with VMs that replicate to the other computer. On Sundays we backup the replicated disks (after creating a shadow copy) for offline storage. We have a problem with one of our VM (running on Windows 2016) which grows the replicated disk (on 2012 R2) far beyond the original. The VM reports disk usage of around 500GB, but the VHDX uses around 900GB of the parent's physical disk, and the replica has grown to close to 1.8TB. When it gets to be a problem for the backup, we remove the replica, and start over. Some things to note:

• At the same time as the backup operation, the VM does some disk intensive operations.
• We do not pause replication while backing up the shadow copy of the replica.
• I've tried to compact the larger replica VHDX, but it did not reduce the physical space on the host disk.

Is there a way to keep the replication from becoming larger and larger, and are we doing something wrong in our backup strategy?

PS: These computers are not on the internet and are not necessarily fully patched.