My iPhone 4S says there's not enough storage?

I only have 17 apps downloaded other than the stationary/un-deletable ones, 240 songs and under 400 photos. I know people with the same amount of GB's and triple the downloads I have, what is wrong? (I noticed this when I attempted to re-download Instagram, it wouldn't load for about 7 hours before I tapped on it and the message appeared)

The nice thing about iPhones is that they can tell you what exactly is using up the storage on your phone without you even needing to plug in the phone to the computer.

1) Open the Settings app.
2) Tap General.
3) Tap Usage.
4) The first thing that will load is a list of items being used in your storage. This might take some time according to how many apps you have and the storage space your iPhone has (e.g.quicker to load the storage data on a 16GB iPhone than on a 32GB iPhone)
5) Take your time and scroll through the list of apps to see how much they use. Realize that some apps that started a small download can take up a huge space as you use it. This is because they use offline storage, or 'cache.'

Many apps use cache (or offline storage) so it wouldn't need to reload the data in the future, it can load them instantly. While it does provide a speed boost when using an app, it can take up a lot of data. To this point there's no way to delete the cache apps use without deleting the app itself and re-installing it. You can delete the app either using the traditional method of tapping and holding an icon on the home screen, or press the delete app button in the Usage list.