It is a simple law of physics. iPhone's shift from anodised aluminiums to glass is because many reasons, but one of it being heat dissipation.

Glass is better conductive than anodised aluminium.

I'm not sure about why data is required for a physics law?

Do one thing - charge your iPhone with case on and measure the back temperature after removing the case when the battery reaches 60%. Do the same thing without the case and measure at 60% again. There will be a difference.

You will get your answer within 2 days. :)