Some scientists say that the enormous Patagotitan mayorum is the largest animal ever to walk the earth. Why are there no land animals of the size of the biggest dinosaurs today? What’s changed about the planet? Presumably it’s still possible for land animals to get that big and the evolutionary advantages of massive size are pretty obvious; who’s going to prey on something like Patagotitan? So why no giant beasts today?
There are disadvantages in growing too large. For large animals it takes time to reach sexual maturity for just a few offspring produced and many young will die before reaching adulthood. It takes a lot of investment by the parents, so such groups will be more vulnerable to adverse changes in the environment and predation compared to smaller animals who can produce more offspring and don’t need to invest as much time and effort in rearing their young. You’re also more conspicuous to would-be prey if you are too big and nimble prey will be able to elude you easier.
Dinosaurs laid eggs which meant they produced several embryos at once, which promoted the chances of survival of their young that much more.
But all the disadvantages you cite would have been around when large dinosaurs were alive. They coped with them, grew huge and still managed to thrive. Why shouldn’t later animals have adopted the same strategy? What is so different about today?
For one thing there was more oxygen about which promotes growth, although this is just a hypothesis and quite controversial.
As I said, dinosaurs laid eggs, therefore, reproduction would not have been such a problem, whereas mammals only make one or two young and if they fall prey to disease or lack of food it takes the population a lot longer to recover. Dinosaurs could simply lay more eggs. I daresay, also, that mammalian young require more rearing than dinosaur young did which puts a premium on the parent’s time and energy. Look at elephants and the way their young have a long childhood requiring much parental care. If a calf dies for some reason it will take a long time to replace that individual.
It’s also possible that sauropods were able to swallow large amounts of food without chewing, increasing body size and permitting the development of bird-like breathing allowing oxygen to be supplied to their bodies more efficiently.
There is no one reason why today’s animals are not as large as dinosaurs but a complex of reasons.
There’s a maximum possible size for land mammals, and it’s around 1/10 that of land reptiles. At least, that’s this person’s theory.
Interesting and goes to show what I said about there being a number of reason why land mammals never got as large as the largest dinosaurs. 
Two factors to conciser: there may have been no need to get that large, and there may have been no way to get that large.
For not need, dinosaurs may have been caught in a feedback loop of bigger predator pushing bigger prey pushing bigger predator pushing bigger prey, and there may be a practical limit on how big a mammalian predator can get, allowing mammalian prey to not need to get particularly huge (and mammals tend to get fast to avoid predators, not get big.)
For no way, mammals, being warm-blooded, may be faced with 1.) not being able to find enough food for a giant body or 2.) not being able to shed enough heat for a giant size, with possibly an evolutionary constraint that prevents them from vastly changing their metabolism. The biggest whales get around this by being filter feeders and by living in water, which sucks out the heat more quickly than air (so much so that they need extra fat.) (Some dinosaurs may have been warm-blooded, but they may have been differently warm-blooded enough that this wasn’t a limit on size.)
They changed the tax codes.
Nowadays, an elephant is already big enough that nothing can prey on it. Why go bigger?
Of course, elephant-size wasn’t big enough for that at the time of the dinosaurs, because there were bigger predators, too. So that suggests an evolutionary arms race of size.
So maybe the answer is just that the evolutionary arms race hasn’t had enough time, since the K-T event, to build back up that far.
There are plenty of reasonably large mammals (less so now, mostly due to human predation).
The true giants among dinosaurs were the Titanosaurs. Perhaps it is simply a matter of random chance why these creatures developed extreme hugeness.
After all, the largest sea creatures ever to live were the Blue Whales - one could equally ask why no predecessor creatures ever grew that large.
Some land mammals grew to pretty enormous size though:
Well, the evolutionary arms race took a different track and becoming huge didn’t particularly help.
Humans prey on elephants. We may have been the leading cause of the extinction of its relative in the Americas, and we may yet send them into oblivion in Africa, as well. But also, there are some lion packs that seem to specialize in preying on elephants.
Indeed, it is quote possible our stone age hunter ancestors played a role in the extinction of the above-noted gigantic species of elephant. I doubt gigantic plant eating dinosaurs would have fared any better, had they existed.
Just an amazing coincidence that all the megafauna dies off as soon as humans arrive …
OK, I should have said that elephants have no predators other than humans. But we’ve been on the scene only for a very short amount of time, not long enough for any significant evolutionary changes other than extinction. And more size wouldn’t help against us, anyway. Elephants are big enough to stop any predator that can be stopped by size, so the point remains that there’s no advantage for them to be bigger.
This site makes the good point that the fact that the biggest known elephant relative and the biggest known rhino relative are more or less tied for size (as best as we can determine) may indicate that this is the size limit for land mammals. (That article also links to this relevant one.)
I thought the prevailing theory on Mammoth extinction was climate change causing loss of habitat and thus food and not humans? As the ice age ended the Mammoths did not adapt quick enough as forests replaced tundra.
It is possible, but a problem with this theory is that numerous other warm periods had occurred during the existence of the mammoths, similar to the end of the last ice age to today, but the mammoths did not die out during any of these.
It is also possible that the warming trend did reduce mammoth populations, but that the “killing blow” was then landed by human predation on this diminished population.