ELI5: Why do hybrid animals like ligers often have health problems while mules do not?

Mules are healthy because their horse and donkey parents have very similar body plans that fit together like matching puzzle pieces, while ligers have health problems because lions and tigers have conflicting growth genes that clash when mixed.

Think of an animal's recipe book, which we call DNA:

• The Mule (The Well-Fitted Bicycle): A horse and a donkey are like two slightly different brands of bicycles. A horse is a mountain bike and a donkey is a city bike. When you combine parts from both to make a mule, the wheels, pedals, and handlebars still line up and work great. Their bodies grow at the same speed and stop at the right time.
• The Liger (The Glitched Toy Car): Lions and tigers are more like trying to force parts from a monster truck into a sports car. They live completely different lives, so their internal instructions clash:

Because the instructions do not balance, a liger grows far too large for its own good. In everyday life, this is like putting a giant truck engine into a small family car. The car's frame will bend, the tires will pop, and the brakes will wear out. A liger's heart, bones, and joints struggle to carry all that extra weight, making them sick and tired.

A mule, on the other hand, gets a balanced set of instructions from both parents, allowing it to grow up strong, tough, and healthy.