On the 11th, the graph may show a zero credit consumption due to a reporting error.
Credit consumption reflects the average database storage cost across all feature services in the organization for a given unit of time.So, if others in your organization have also published feature services, it would be reflected in the credit consumption. If you are the only one publishing, I'm not sure why you're seeing fluctuations. How much fluctuation is there?
The amount of data you upload is not equal to its size in the database. So uploading a 500MB dataset can yield a 1GB set of tables in the database. Many tables get created to manage the service, it's not just the data itself.
Thanks,
Mike