How to fix Gallery images deform?

How to set product gallery thumbnails pictures to be constrain proportions effect in single product page view? How to set/adjust the images size as preferred? The current gallery pictures in single product page are deformed before enlarging view and need to be fixed. Hope this can be set in child theme or custom CSS plugin if not a part of the theme features.

See problem page:
https://www.chopsticksfactory.com/product/32oz-carryout-white-diamond-bowl-wlids-300ct/

Please help. Thanks in advance!