From what I can guess the green background of the felt you take your pictures with uses a lot of colors and shades. That probably results in an image that doesn't compress as well while still leaving a good quality image with the compression levels we use.
One thing you could try would be using a more solid color material. Perhaps a white sheet. If you did that and the background color was more of a solid it would compress perfectly leaving a almost all the rest of the space of the image for the item. So you'd end up with a sharp image, and still have it be a small # of bytes so that it downloads fast for buyers viewing your items.
That's the goal you want to shoot for. Great looking images that are sharp, but are still as small as they can be # of bytes. Since they only need to download it if they click the supersize link though it's probably ok for the image there to be a little bit bigger.