Magento makes 3 copies of your files, one for the thumbnail, one for small, and the Normal sized one. if you are using cache this means per 1 product you could have up 9 copies, per single image added per product.
I ran an image optimizer to try and reduce my files sizes, be cautious as mass jpg optimizers don't always reduce the files and can create colour table errors, as I experienced recently with some jpg's that where 5000x3000px, 10mb per file, it reduced them by 12% but the colours where all messed up.
Follow the Magento guidelines for image sizes dimensions, yes will take a long time to produce, for the 3 size types, but if you create for all 3, the thumbnail and the small should only be a few kb each where as you main image can be 500kb avg. and vastly reduce your taken hdd space.