I have not yet found a lot of use for Azure Drives given some of the limitations that they have and the other storage options that are available, so I have only done some playing with them, not actually used one in a production environment.
With that said, based on my understanding, and the description you give in your question about only being charged for the amount of content stored on the drive I do not see any issue with creating a large drive initially and growing into it in the future.
Hope that helps some, even if it is just a - yes I think you understand it correctly!