system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are ligers typically much larger and heavier than both lions and tigers?
gpt
Built into your genes is code to *stop* growing. That's important because, for example, just making your heart *bigger* does not make it *better* at pumping blood. It takes additional muscle and different structure to be efficient. So if you get too big, your heart can't keep up with the need to pump blood farther and farther away from your lungs, and it gives out.

As a result, humans with gigantism tend to die in their 30s and 40s to heart failure. Similarly, ligers tend to die young relative to their parent species due to heart failure.

Anyway, it just so happens that the gene to stop growing in lions is found in genes that come from the mother, while gene to stop growing in tigers comes from the father. (More accurately, they are *activated* based on the parent's sex.)

Ligers have lion fathers and tiger mothers. So they get neither of the species' genes to stop growing, so they don't stop growing. Tigons - hybrids with a tiger father and a lion mother - grow to be about as big as, well, lions and tigers. They don't get like, *extra* stopping genes, they just have two sets that turn on about when they're supposed to so they get as big as either of their parents should be.

Side note, the portmanteau word for a hybrid depends on which is the mother and which is the father. Fathers form the first part of the word, mothers the second. For example, a Zorse is a hybrid male zebra and female horse; a hebra is a hybrid male horse and female zebra.