Why Is My Responsive Popup Wider Than the Screen On Mobile Devices?
Your popup is set to Responsive Sizes under the Display Settings Pane, but the popup itself is wider than the screen on small devices, making it appear non-responsive.
Your popup's content (the part we have no control over) is wider than the screen, thus the popup had to expand to fit content inside of the popup. The popup itself is indeed responsive, but the content inside the popup is not responsive.
Make sure the content inside of your popup is responsive by applying or adding responsive styles to said popup content. Check out this CSS Tricks article for some reference and help to help make your popup content responsive.