https://code.google.com/p/gmaps-api-issues/issues/detail?id=3117#c12

google-maps--issue-3117.js

// From https://code.google.com/p/gmaps-api-issues/issues/detail?id=3117#c12

function googleMapsFitBoundsAndZoom(google, map, bounds) {

  function myFitBounds(myMap, bounds) {
      myMap.fitBounds(bounds);
  
      var overlayHelper = new google.maps.OverlayView();
      overlayHelper.draw = function () {
          if (!this.ready) {
              var zoom = getExtraZoom(this.getProjection(), bounds, myMap.getBounds());
              if (zoom > 0) {
                  myMap.setZoom(myMap.getZoom() + zoom);
              }
              this.ready = true;
              google.maps.event.trigger(this, 'ready');
          }
      };
      overlayHelper.setMap(myMap);
  }
  
  // LatLngBounds b1, b2 -> zoom increment
  function getExtraZoom(projection, expectedBounds, actualBounds) {
      var expectedSize = getSizeInPixels(projection, expectedBounds),
          actualSize = getSizeInPixels(projection, actualBounds);
  
      if (Math.floor(expectedSize.x) == 0 || Math.floor(expectedSize.y) == 0) {
          return 0;
      }
  
      var qx = actualSize.x / expectedSize.x;
      var qy = actualSize.y / expectedSize.y;
      var min = Math.min(qx, qy);
  
      if (min < 1) {
          return 0;
      }
  
      return Math.floor(Math.log(min) / Math.log(2) /* = log2(min) */);
  }
  
  // LatLngBounds bnds -> height and width as a Point
  function getSizeInPixels(projection, bounds) {
      var sw = projection.fromLatLngToContainerPixel(bounds.getSouthWest());
      var ne = projection.fromLatLngToContainerPixel(bounds.getNorthEast());
      return new google.maps.Point(Math.abs(sw.y - ne.y), Math.abs(sw.x - ne.x));
  }
  
  return myFitBounds(map, bounds);
  
}