RCD popup size problem

I am having trouble with popup sizing with RCD. When I resize the browser window to phone portrait view, the popup displays as I would expect (based on the phone portrait page size). However, when viewed on an actual phone (or using Chrome's device emulator), the popup displays much smaller. I have attached two screen shots depicting the issue.

I have tried specifying the popup window size but that had no effect.

Am I missing something here? Or is this just how it works???