Magento 2 - Keep Product Image Aspect Ratio

You can use below code.

<?php
    //$image = 'category_page_grid' or 'category_page_list';
    $_imagehelper = $this->helper('Magento\Catalog\Helper\Image');

    $productImage = $_imagehelper->init($_product, $image)->constrainOnly(FALSE)->keepAspectRatio(TRUE)->keepFrame(FALSE)->resize(400)->getUrl();
?>

<img src="<?php echo $productImage; ?>" />

You can set "Keep Product Image Aspect Ratio" in magento2 like magento 1.x using image helper.

You can use image helper like this in list file : app\code\Magento\Catalog\view\frontend\templates\product\list.phtml

$_Imagehelper = $this->helper('Magento\Catalog\Helper\Image');

<img src="<?php echo $_Imagehelper->init($_product, 'small_image')->keepAspectRatio(true)->resize('height', 'width'); ?>" />