I know that the Chrome browser compresses data. If you are using an iOS device, most likely you would be using Safari browser.
It is also possible that you use different apps on each platform and those apps uses different amounts of data. Not all apps are available on both platforms and you may have to find substitutes when switching platforms. Those substitutes may use different amounts of data.
It is also possible the settings on apps are different on different phones. For instance, if you configure an app on one phone to do background updates every 15 minutes and the app on the other phone to do background updates every 2 hours, the one updating every 15 minutes will use more data.
The only way to know for sure is to use some data monitoring tool on both phones and do a comparison. I know they exist on Android. I'm not sure about iOS.