Hyper-V snapshots growing until disk is full?

I have a legacy application that requires Windows 2000 server.  Last September we virtualized the box on Hyper-V.  It was the only VM running on the host.  This past weekend, the host ran out of disk space because two Hyper-v snapshots very suddenly began growing in size until the available disk space on the host was full.   The snapshots in question had both been taken over 4 months ago.   I was unable to delete the snapshot files because the merge process was failing do to lack of disk space.

In order to get the VM back online I was forced to move it to a new host with more disk space.

I understand that my setup did not follow best practices, but I'm looking for insight into why the snapshots began increasing in size.  Any help would be appreciated.