To confirm, have you made any recent changes to your theme settings/CSS code? There may be a CSS rule affecting the size of your product images. You can try inspecting the code and look for any rules related to image size/dimensions.
Could you also confirm the product image size? Product and collection images can be any size up to 4472 x 4472 px, or 20 megapixels. Typically, we recommend using 2048 by 2048 pixels for square product photos. You can use the Shopify Image Resizer toolto ensure your product photos fit these dimensions. You can try removing the image, resizing it and re-uploading it to see if this fixes the sizing issue.
Please let me know if this has worked and if you have additional questions.
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