And this is exactly why it is happening. The code in product template is (or similar to this)
Quote:
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{include file="product_thumbnail.tpl" productid=$product.image_id image_x=$product.image_x image_y=$product.image_y product=$product.product|escape tmbn_url=$product.image_url id="product_thumbnail" type=$product.image_type}
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Note "image_x" and "image_y" which are assigned actual product image dimensions (main image not thumbnail). These values are passed to product_thumbnail.tpl to use as image dimensions. But you changed the code in that template to use exact values (150/150) so the actual image dimensions are ignored.