Hello @Christopher1976,
Welcome to the community!
It depends on the phone you have, if you have an iPhone its know that iPhones use a lot of data.
You also need to check your settings to see if you have data assist/download boost turned on if you disable it so no data will be used when on WiFi.
You can also check your usage by app to see if there is a rough app that is using up your data, see here on how to check for iPhone and here for Android.
