Why would you say that?!?

Continuing on the theme of poorly chosen everyday phrases....
"I've gotta pee like a race horse!"

Why oh why is a race horse the standard for urination? Does a race horse go more often than any other horse? Did Secretariat have a smaller bladder than Mr. Ed? Has somebody taken the time to measure the quantity that a thoroughbred pees and compared it to other animals? (I don't doubt it!) Is the size of a human bladder the same as a horse's? And furthermore, what do the makers of products like Detrol think of this?

"Side affects are mild, and may include dry mouth, constipation and peeing like a race horse..."