Mules are slightly smaller than their horse parent, and when they grow they share traits of both a donkey (the long ears), and the horse (color, tail, etc.). A Mule in the grass.
For example, a 2014 Ford F-150 is considered a half ton truck but most variants can carry a lot more than 1,000 pounds in the bed. Since these terms are probably not going away any time soon, it's best just to think of them as smaller, mediumest, and more than medium.. (Pause for laughter).
Unfortunately the long answer is more complicated and is plagued by a history of shifting standards and marketing hype. Let me explain. Back in the early days of trucks, the terms simply described how much a truck was rated to haul. You bought a half-ton truck and you can haul up to 1,000 pounds of cargo and passengers.