function _increasePercent(size) {
    for (var i = 0, elmsLength = this._targetElement.length; i < elmsLength; i++) {
      var currentElement = this._targetElement[i];
      if (currentElement.hasAttribute('data-progressjs')) {
        var percentElement  = _getPercentElement(currentElement);
        var existingPercent = parseInt(percentElement.style.width.replace('%', ''));
        if (existingPercent) {
          _setPercentFor.call(this, currentElement, existingPercent + (size || 1));
        }
      }
    }
  }
Increase the size of progress bar

@api private
@method _increasePercent
@param {Number} size