I have an iPhone with less memory than bought?

I have an iPhone 4S with 16GB storage space. I have checked my usage settings, and although only 13.3GB are full, I apparently have no more storage space.

The 16GB (or whatever size of memory) is also used by the operating system in the phone.

I've just done a full reset on a 16GB iphone for a friend; with nothing at all added or installed, the free storage capacity is 13.5 GB.

Yours is legitimately full!

Your iPhone has 16GB but some is used by it's OS, you can use what is left.

It is important to know that you never get the full memory. For example, on an 8GB SD card you won't get the full 8GB, you'd probably get around 6GB. There's nothing to worry about, your iPhone is legitimate, and there's nothing wrong with your phone. I brought a 64GB SD card and you don't get the full 64GB because a bit of memory has to be used for the system, etc…

Take care, have a great day.

That's how it is. What you can't access/use is what is used to store iOS.

For the record hard drive based devices are like that. Don't get all the space as it is used to store what OS it has.