If you are saying (as you weren't very clear) that you are only able to access 8GB of space on the device when it is supposed to have a total of 16GB of space, then you would be both right and wrong in your information.
Where you are right is in stating that the device has a total of 16GB of space. However, where you are wrong is in thinking that you would have/could have access to all that space.
See that is the full total space for the phone. Now long before you even get the phone much of that space is taken up by the OS and the preinstalled apps. So you only have access to what is left over after they are installed. Then what you have left over, that you are able to use from the moment you got the phone, is taken up by apps you add, any anything else at all you put on the phone. All data. Text messages, photos, music, videos, new apps, data the apps add/need, sites you visit, contacts, call history, etc. All that takes up more and more space.