system
Given the following post, respond as if you're commenting on Reddit:
human
How where Jurassic Herbivores (like the Stegosaurus) able to physically sustain themselves with only leaves which are so low in Calorie?

I know they where basically eating for the entirety of the day, but I still don’t understand how such a big animal could sustain itself with only plants when even a koala eats all day just to sustain.
gpt
In terms of efficiency, getting bigger is better so long as there is an available food source.  Larger animals have lower area/volume ratios, meaning they need to spend less energy to stay warm.  So if you double your mass, the mass of food you need to eat increases but does not double.  Big animals are also harder to prey upon, which is a valid defense strategy that doesn't involve spending time or energy on other tactics (horns, big teeth, armor, etc).  It is harder to move around when you get big, but this trade-off may be worth it for the positives up to a point.  The natural stopping point will vary depending on the ecosystem, but one of the hard stops involves gravity and the maximum strength of bio-materials.  Until nature evolves a stronger type of bone, land animals can't get so big that bones won't support them.  

You can see this in action comparing mice and elephants.  You could drop a mouse off a skyscraper, and when it landed it would be just fine, so long as it didn't freeze to death of die of panic.  Its body is so light that impacts from free fall are not enough to injure it.  On the other hand, elephants can break limbs just by tripping, and are completely incapable of jumping even an inch or two without risking death.  Something much bigger than an elephant wouldn't even be able to stand up without shattering its legs. 

The payoff for the elephant's size and fragility is it's efficiency.  An elephant eats a lot of food every day, but that food adds up to only a tiny portion of its body mass per day.  On the other hand, tiny animals like mice need to eat a significant portion of their body mass every day, because they need to use tons of energy to keep warm.  Hummingbirds are really scraping the bottom edge of the possible size for warm blooded animals, and they need to spend so much energy keeping warm that they must eat more than half their body weight per day.  Imagine if humans had to eat 60-100lbs of food per day in order to not starve to death.  

In the water, even the movement penalty is gone.  The bigger you get, the less energy you need to use to move around.  Since there are basically no downsides to being big in the water, things which adopt the "get bigger" strategy don't hit any sort of stopping point and continue to get more and more gigantic.  Whales are a good example of this, as well as sharks and the aquatic dinosaurs.  

Another natural limitation involves the circulatory system.  The bigger an animal, the hard it is to get nutrients distributed to all of the internal cells.  You can therefore see size limitations for different sorts of circulatory systems.  Animals with no circulatory system must remain quite small.  Insects have a sort of "circulation goop plus stirring mechanism" system which allows them to get a bit bigger, but the upper limit on insects is probably under a pound.  Advanced circulatory systems with arteries, veins, and multi-chamber hearts allows animals to get huge (whales and dinosaurs), but there is probably some upper limit on those as well.  Interestingly, the size limits for different systems depends on the availability of nutrients.  Larger animals cannot be found where nutrients are sparse, such as high up on mountains where there is little oxygen.  Millions of years ago, there was actually more oxygen in the air, which allowed bigger creatures with cruder circulatory systems.  For instance, there were giant dragon flies and other insects which were 10x or more the size of today's insects.