By default, our themes set a maximum height for your product images so the grid displays in a uniform manner regardless of the sizing of the images. Since many merchants do not have product images that are all the same sizes, this setting ensures that your collection page grid looks clean.
If you'd like to override this feature or make finer adjustments, you will find the following setting in your theme settings tab in our ShowTime and Galleria themes:
You can control the maximum heights of the product images with this slider. Generally speaking, if you want larger product images, you should set a higher value in this range. This affects images on collection pages, your collection list page, and also featured products/collections on the home page. If all your featured product images have the same aspect ratio (which is optimal), then you should uncheck this box and you can ignore this setting. Your images will show as large as they can in the grid and will appear perfectly aligned. If your images are all shapes and sizes, you can also leave this box unchecked, but your grid may look uneven.
Of course there is no harm in trying all the options to see which settings are most pleasing to you and your particular products.