Yes -- because, just like KM, the workflow is triggered when the folder is changed (the new file is created) not when it finishes changing (writing is complete).
The simple way round that is to add a "Pause for n seconds" Action to your Hazel workflow, before the "Move" Action. You can then optimise the flow by changing the "Pause" for something keeps checking the file size, only letting the flow continue once the size stops changing.
How to do that in Hazel is probably best asked on the Hazel Forums -- but have a search first as this is a common problem. It's also been covered here many times, most recently here.