If products have featured images of different size the product grid items will have different height and the inner content, such as title, price and “Add to cart button” would be on different levels. The best solution is to upload images of equal size and proportion, however, considering a great deal of products on the site, such a solution cannot be always applicable.
In order to settle the issue, navigate to Appearance - > Customize -> WooCommerce -> Product Image and find the size option for product page image as well as images in a grid: