Let science explain:
It has to do with the design of the bladder," Hu tells Popular Science. In other words, it's all in the urethra, the bodily pipe that moves urine from the bladder out of the body. (In dudes, it also ferries sperm.) "It's really important for making the bigger animals eject fluid faster," he says. "It’s the length of the pipe that matters." Because larger animals seem to use gravity, rather than the muscles of the bladder, to keep their flow going, the longer the tube, the faster a single molecule moves through it. The diameter of the urethra matters, too, by increasing the volume of liquid that comes out every second—in the elephant's case, that 1.5 gallons.
The ratio between the length and diameter of the urethra was nearly constant for the animals Hu and his team studied: 20 times longer than it was wide. The elephant's drainage pipe of a urethra (1 meter long, 10 centimeters in diameter) has exactly the same aspect ratio as the mouse's, which is about the size of a thick staple (1 centimeter long, 1 millimeter in diameter).
Note - there is a video on the web site, I see no reason to copy it here - you can check it out if you want to... I was all good until the elephant did a wizzer AND a boom boom at the same time. In slow motion that was not pleasant.