A couple of thoughts:
Do you have any snapshots? Turning on Bitlocker rewrites every disk block in your VM as it initially encrypts the drive. Those rewritten blocks have to be maintained in the snapshot. So if you have a snapshot of a 50GB drive taken before you started Bitlocker, expect to see another 50gb (give or take) to be consumed. That space will be consumed until you delete all the snapshots taken before Bitlocker was enabled.
If you did not tell Bitlocker to encrypt used space only, the entire virtual hard drive will be encrypted. If your virtual disks were not configured to pre-allocate disk space the Virtual disk file on you Mac will be expanded to it’s maximum size as a result.