iframe-fullscreen-sizer.js

// Make sure that iframe resizes correctly on fullscreen exit
// (ie, make Vimeo or YouTube videos return to the iFrame bounds on fullscreen exit)

var isFullscreen = false;
var videoSizer = null;

function onFullscreenToggle(){
  var isFullscreen = document.fullscreenElement ||
  	document.webkitFullscreenElement ||
  	document.mozFullScreenElement ||
  	document.msFullscreenElement;

  if( !isFullscreen ){
    // Resize after brief delay to account for zoom-out animation 
    setTimeout(function(){
      sizeVideos();
    }, 200);
  }
}

function sizeVideos(){
  // Size your videos here!
}

// Fullscreen event handlers
//document.addEventListener("fullscreenchange", onFullscreenToggle, false);
//document.addEventListener("webkitfullscreenchange", onFullscreenToggle, false);
//document.addEventListener("mozfullscreenchange", onFullscreenToggle, false);
// Or in jQuery style:
jQuery(document).on('mozfullscreenchange webkitfullscreenchange fullscreenchange', onFullscreenToggle);