Believe it or not, tigers are not the largest big cat. Ligers are (you might remember ligers from Napoleon Dynamite). Why? It has to do with the weirdness that occurs when you hybridize across two lineages which have been distinctive for millions of years, but not so long so as not to be able to produce viable offspring (in fact, many ligers are fertile as well).
Here’s the explanation:
Imprinted genes are under greater selective pressure than normal genes. This is because only one copy is active at a time. Any variations in that copy will be expressed. There is no “back-up copy” to mask its effects. As a result, imprinted genes evolve more rapidly than other genes. And imprinting patterns — which genes are silenced in the eggs and sperm — also evolve quickly. They can be quite different in closely related species.
Lions and tigers don’t normally meet in nature. But they can get along very well in captivity, where they sometimes produce hybrid offspring. The offspring look different, depending on who the mother is. A male lion and a female tiger produce a liger – the biggest of the big cats. A male tiger and a female lion produce a tigon, a cat that is about the same size as its parents.
The difference in size and appearance between ligers and tigons is due in part to the parents’ differently imprinted genes. Other animals can also hybridize, with similar results. For example, a horse and a donkey can produce a mule or a hinny.
Imprinting generally emerges due to competition between the interests of males and females within a given species because of complex social structures. What’s good for father may not be good for mother. Lions live in prides, while tigers are relatively solitary. Apparently lionesses may mate with multiple males, so any given male has genes which tend to encourage growth in his own cubs as to as maximize his genes’ share of finite resources in a competitive environment.
In contrast, the female’s genes tend to fight against this tendency, because she’s equally related to all the potential cubs, and so wants to equally distribute resources as to maximize the number who might survive. Tigers are not subject to this dynamic. A tigress mates with one male, and so he is equally related to all the cubs. His genes would not want to “encourage” growth because there isn’t competition between cubs from the male perspective, they’re all of a piece. So the female does not need to evolve anti-growth imprinting defenses.
You bring a female tiger, who has no defenses against paternally inherited genes which tend to encourage growth, with a male lion who will contribute exactly those genes. And voilà , you get the liger, whose growth is a consequence of this asymmetry at the endpoint of different evolutionary histories.