Construct 3 export index.html is different than Construct 2's and seems that there is stuff in some script file that does resizing. Maybe Ashley can advise on how to accomplish this.
Anyway, when you view HTML5 exported game it will resize smoothly as you resize browser window. How to do this when the game is embedded within an iFrame ( now you are resizing the page that contains the iFrame ). For mobiles this seems like a necessary thing.
I have found a few different solutions, but there are problems with each, like often it will resize when you make your window bigger, but will stay that way if you flip the phone, or won't resize back. I have noticed this a few times even when viewing games on iPhones normally ( not iFramed )