This problem is specific to already well compressed jpegs. When Magento is re-saving them to the cache, it increases the file size. In some cases almost doubling the size compared to the original. Obviously, the bigger the image, the greater the problem in absolute terms (of lost bandwidth/ performance).
This is somewhat infuriating! How can I stop Magento messing with my images; and why on earth is it adding pixel data?!
I’m only allowed to post links to one example. However, I could pick any image, it is increasing them all:
Original Image - https://www.logsdirect.co.uk/media/catalog/product/p/e/peat-pallet-french-bottom_1.jpg (75 KB)