You may have noticed the following setting in your Galleria theme settings tab:

In order to keep products on the collection grid looking tidy and uniform regardless of the size of your images, the theme uses some fixed maximum heights for the product images shown there.  You can control these maximum heights 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 will 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.