Although I agree that it's probably best to resize your images, I think it's possible to do what you're asking.
I think the following code will work in products.tpl (I haven't tested it). If you're wanting to make the change to the single product page, you'd use $product.image_x.
Code:
{math equation="x/3" x=$products[product].image_x assign="proportional" }
<a href=product.php?productid={$products[product].productid}&cat={$cat}&page={$navigation_page}>{include file="product_thumbnail.tpl" productid=$products[product].productid image_x=$proportional product=$products[product].product tmbn_url=$products[product].tmbn_url}
{$lng.lbl_see_details}</a>
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