The autoupdate function for apps can be turned off in Settings/Store.
If you are worried about the data traffic, switch to WiFi while updating your apps, or update them on your computer and sync your phone after that.
Are you using youtube a lot? This can result in heavy amount of mobile data transferred to your device, if you're using mobile data service instead of WiFi.
I almost forgot about this feature, because I never use it, to be save that I'm not using more than my limited mobile data
Message was edited by: Ingo2711