Click Settings app.
iPhone settings are displayed. Click Cellular.
Cellular settings are displayed. Scroll down the page till you see 'Use Cellular Data For:'.
As you can see, all the apps are enabled to use cellular data hence eating up mobile bandwidth. Slide the toggles for the apps that you want not to use cellular data to the left and disable them. For example, 24me, as shown below.
Now the app 24me will not use cellular bandwidth whenever the iPhone connects to mobile data. Similarly you can turn off other apps as per your will, specially those which you do not use often.