The virtual machines are stored in Azure storage itself. They take up that amount of space in the Blob storage. If for example, your VM is 30 GBs bug, it will occupy 30 GBs in the Azure blob storage and it will cost you accordingly.
You can check the pricing for Azure blob storage, as it keeps getting updated with time to calculate how much you may be spending.
And also check out Intellipaat's Azure Administrator Certification and become an Azure expert