An elephant takes the same amount of time to empty its huge bladder as a cat - despite holding 3,600 times more urine. That's a whole lot of pressure.
The controlling factor of urination duration is the length of the urethra, which gets longer at a predictable ratio as an animal gets bigger. All animals have urethras of the same aspect ratio: a length-to-width ratio of 18. This is rare among animals. Usually, body parts change in relative size, such as the eyes and brain.
As the urethra gets longer, the effects of gravity increase and there's more pressure pushing out the urine.
Source: http://www.sciencealert.com/most-mammals-take-21-seconds-to-pee-regardless-of-their-size
