Something is eating up my internet allowance! How come if I download a file of 200mb my internet usage is so much more?? Once got 3 files total 600mb and couple gigabytes gone!! Do I have a virus? Is someone piggybacking on my usage? Ta
You need to monitor the phone's data usage and see what apps are using it most,
Some phones have a basic monitor built in but there are also plenty of (often free) apps that will do a more detailed job.
Once you know what apps are using data you can see what steps you need to take.
It's unlikely that anybody is stealing bandwidth on you unless you somehow have an unsecured open hotspot enabled all the time - if thats' even possible.
Hope this helps.