Ollie here from the Shopify Support team.
I appreciate the image that you have added to your post as that is super helpful. Would you let us know what theme you are using?
One method to have the images appear as the same thumbnail size is to upload the images as the same size. You can use the Shopify image editor to resize the images that have been added to your products. Your product and collection images can be any size up to 4472 x 4472 px, or 20 megapixels. Alternatively, we can also look towards using a third-party application that can assist you with resizing the images on the online store. I created a list of recommendations from the Shopify App store, feel free to take a look:
Let me know what you think.