Why canivores like tigers are not as bulky as herbivorous animals like elephant

I believe being large comes at a price. The larger you are the slower you become, and so you're not as quick to hunt. Think of the male lion, it mostly eats what the female lions catch, which are thinner.

THANKS A LOT

They are so big because they have to eat so much to keep themselves going. And, considering prehistory to the present, they apparently get enough proteins and nutrients to keep going since they have been doing so for Millennia. Their bodies are designed to get what they need from plant life..else they would be carnivores.

Entropy.

When energy changes forms, some is always lost. This is what entropy is. Most life on earth is dependant on the sun as the source of energy. Plants take this energy and turn it into carbohydrates and proteins. However, not all this energy is used - a small amount is changed to heat. Animals which eat plants are the next organisms to capture this energy, but again some is lost. The result of this is that there will always be more plants than the animals which eat them. In turn, there are even less animals which eat plant eating animals. Carnivors get very little of the original energy which the sun produces.

The availability of energy determines how large an organism can grow. Trees get very large because they absorb energy all day long. Deer eat large meals every day, but do not feed as long as energy is available. This makes deer smaller than trees. Wolves eat only every other day and are smaller than deer. This is the general trend amoungst land plants and animals.

In the ocean, plants are single cells, but the mass of plant material itself is even greater than all land living plants. The cells are eaten by small crustaceans which are also far more numerous than plant eating animals on land. These tiny crustaceans form the diets of enormous animals such as whales and giant sharks. These animals are far bigger than land preditors, but are actually rather uncommon.

Well, there was an interesting study done regarding dinosaurs edit: the study INCLUDED dinosaurs, it looks at mammals as well (clearly not mammals, but the principle behind the study still can apply) that stated, "For a given land area, the top herbivore will be larger than the top carnivore by a factor predictable from the greater amounts of food available to herbivores than to carnivores".

They also stated that, "Within a trophic level and for a given area of landmass, top species that are ectotherms will be larger than ones that are endotherms, by a factor predictable from ectotherms' lower food requirement."

So it seems that the principal decider of maximum size for the "top species" is the availability of food.

Now, this study was referenced in the book "Whales, Whaling, and Ocean Ecosystems" on page 192 where they use it to prove their statement about mammals being larger with greater food abundance. "In general, we find that, within mammalian groups, hunters gorging on relatively large prey are smaller than grazers that consume smaller prey items over multiple feeding bouts. Consequently, some of the largest mammals feed on some of the smallest prey or plants"
"For mammals in general, the quality of the diet and accompanying digestive physiology also influence optimal body size. As a result, the maximum body size of carnivores is often less than that of herbivores".

What they were referencing when they said "quality of diet and accompanying digestive physiology" was the fact that with a poorer quality diet, the animal needs larger digestive organs to be able to utilize the food efficiently (think about how cows can eat just grass and grow, because they need a huge rumen - and microbes - to ferment the grass, whereas wolves eat a high quality meat diet so they have a very simple and, relative to their body size, a smaller digestive system).

J'adore's answer is right to a point, but there are theories on this. One thought is that meat protein is efficient,or compact versus a plant eater needs more food bulk, more processing organs, so he needs a bigger body. Another reason is that a carnivore must hunt and needs an efficient body to overcome it's victim, yet may not always have food available. Plant eaters generally are foraging all the time and can evolve to grow over generations.