I lived in Riyadh for 17 years. Almost all the really long distance flights left between midnight and 3 am so they could carry enough fuel and cargo/passengers.

Hot fuel takes up more space, but that is a minor problem. As the articles have said, hot air and altitude (Riyadh is about 2,000 feet) lower lift. You have to reduce load as you can't reduce fuel on really long flights. And less load equals less profit.