MSDN article paragraph:
All sites in a site collection are stored together in the same SQL database.
Keep extremely active sites in separate site collections.
Because all content in a site collection is stored in the same content database, the performance of database operations — such as backing up and restoring content — will depend on the amount of content across the site collection, the size of the database, the speed of the servers hosting the database, and other factors.
Isnt it that the web application is assigned a content db (when creating a new web app we assign a content db to it) and thus all site collections reside in that same db?