Q:

How can I control a content database size in sharepoint 2013

My case is I have one web application which has only one site collection, for example, http://examplesite . This site collection has a content database, for example, wss_content. Then I created a new content database called wss_content_2015. I used 'move-spsite http://examplesite -DestinationDatabase WSS_Content_2015' to move from old to new content database. my questions are: 

Because I have only one site collection, therefor I expected the
wss_content should be empty after I executed the command, but it
turned out there is still one third of whole database left, why?
if I upload the documents to the same url http://examplesite now, where these documents will be saved? in wss_content or wss_content_2015? is there a way which i can control how to save uploaded documents?

A:

when you move a site collection from one content database to other content database, it moved the site collection after that size of the source content db(wss_content) still be with same size, as that space showed as Unused(white space) in the content db. In that case you have to shrink the content database to get the space back.
Now site collection moved to new content DB(WSS_Content_2015), so all the operation will be performed on this DB i.e upload the documents or creating sites etc.

