I've not tried the tool suggested by Heath, but I suspect it won't tell you what you want to know about specific feature layer storage. This is because all feature layer are stored in a single database, and the database currently doesn't allow us to calculate storage on a per layer basis.
Look for feature layers that have attachments. Picture from a camera can be several megabytes. Thus the storage of these features can add up quickly. If your feature layer was created before July 2014, it will store attachments as blobs in the database. After July, we made a change to AGOL to store attachments as files, so the credit usage is much less, as file storage is much less.
Hope this helps.
Mike