How the hell would they allow you to use a phone on a plane? With WiFi enabled?
Check your iPhone. When you turn on "Flight Mode", Wi-Fi switches off but can be switched back on without exiting Flight Mode. Your phone is supposed to be completely off during take-off and landing, but once in the air, you can turn it back on and use Wi-Fi, as long as you had put it into Flight Mode before you turned it off (the cell functions have to remain off at all times).
They wouldn't offer in-flight Wi-Fi if it was a problem. One assumes they know what they're doing.
The different pricing seems like a scam though. Charging more for the same service because your screen is bigger. Larger devices are generally able to consume data quicker, but unless the service is some form of "unlimited data", this sounds like a rort.