Why couldn't they be twice as bigger then average size? Like most average cats are 12-15lbs if they were 2x as big at least 24-30lbs wouldn't they be stronger to defend themselves from fox, raccoons, possums, and others?
They jaw strength will be munch stronger and more durable to take attacks from enemies so they try to resist?

Hi Silverio,

One must remember that evolution is a game of trade-offs. Imagine that a biological organism is an organisation with a finite set of resources at a given time. While yes, being bigger and stronger would of course mean that the cat could better defend itself, the biological energy and material needed for that would mean that other processes would be neglected.

An example to illustrate this is African prey-predator relationships. The cheetah chasing the gazelle will suffice. The ceetah has evolved to deliver a huge burst of speed to catch prey and yet cannot maintain this over long distances due to energy loss. One might ask, why they dont evolve to run longer. Well, they do in a sense but in order to run at that speed for that long, the cheetah would need to have a heart the size of a car and legs as powerful as an elephant. This leads to  hosts of other problems.

Similarly, the gazelle can run away quickly due to its nimble legs. However, this same trait also leads to its downfall as the nimble limbs which afford it such speed are also fragile and prone to easily break. Thus, there is a trade off between speed and strenght as one cannot merely increase both. Increasing the strength of the leg leads to loss of agility and increasing speed to evade the cheetah leads to increased chance of breaking a leg and ending up as dinner.