Issue with thumbnail sizes

Seem to be having an issue where the thumbnail sizes are inconsistent. I can see that they are being resized based on the overall width of the slick-track, so for galleries with many images the thumbnails are tiny. How can I make these consistent? Below are 2 examples, the first the thumbnails are correctly sized, and the second they are not.

https://www.staffordjewelers.com/shop/ritani-side-stone-engagement-ring-r2716/
https://www.staffordjewelers.com/shop/ritani-halo-set-engagement-ring-r1323/

The page I need help with: [log in to see the link]