The capacity is not changing 'rapidly', instead the whole VDO device is thin provisioned. Let's assume a 1TB harddisk, then when creating a VDO device ontop, you can have that directly appear as 3TB, so 'thin provisioned'.

The filesystem ontop always sees 3TB, unless you explicitly modify the VDO device. Of course, you have to monitor the VDO status tightly: if you happen to store data on the filesystem which is absolutely unique, has no zeros and is uncompressable, then dedup/compression can not do anything. The VDO device can only consume up to ~1TB of such data. Your monitoring should detect the low space on the VDO backend before, and you should then either stop writing or extend the backend device.