If the size is specified in the HTML, then it is honoured if it makes the image
smaller. As pdurrant says, this is probably what is happening here.
Also, on the Kindle most images are automatically doubled in size. The exception is if you have hisrc specified, so I am guessing that this is the case here. Usually you are better off without hisrc and without explicitly setting the size in the HTML. See
Image optimization for Kindle 2.