It's true that there are animals today, notably the blue whale, that's the largest creature ever to inhabit the earth, that are bigger than dinosaurs but no creature that currently walks on land gets anywhere close to the size of the super dinos.

The largest recorded species of dinosaur probably is Amphicoelias fragillimus which measured up to 60 meters and weighed 122 metric tons. By comparison the African Savanna Elephant, the largest modern land animal clocks in at just 3 to 4 meters tall and 5 and a half tons.  Both herbivores. So why aren't there land mammals today that are as big as the dinosaurs?

The fossil record gives us some clues as to when and how if not exactly why mammals hit their limit. After the last of the dinos disappeared around 65 million years ago, the size and diversity of mammals went totally nuts...until it didn't. Around 30 million years ago, a kind of giant rhino giraffe beast topped out at 5 meters tall and 7 and a half meters long grabbing the record for the biggest mammal to ever walk the earth.  

Scientists think that there are several limiting factors that have kept modern land animals from approaching Supersaurus sizes. First, mammals give birth to live young.  A mama dinosaur can just lay a bunch of eggs and walk away but a mammal has to gestate and nurse and rear babies at a great energetic cost.  Large mammals also have long periods of gestation  Giraffes and rhinos gestate for about 15 months and elephants are pregnant for nearly two years.  And the longer a pregnancy is, the more dangerous they are for both the mother and the young so a mega herbivore would probably experience an unfeasibly long and dangerous pregnancy.  

Another theory suggests the dinosaurs grew so large because of Earth's climate and ecology at the time. A warmer muggy climate with a higher carbon content in the air made the vegetation grow off the hook.  Sauropods basically lived in a massive smorgasbord and they may have got so titanic simply because they could.  But the environment changed over time and herbivores today don't live in such prime conditions.  

Reason number 3: today's land mammals are further limited by their skeletal systems.  Sauropod fossils show evidence of avian like air sacs that made for much lighter skeletons than you'd expect in animals so large.  Some of these dudes were 30 meters long but only weighed an estimated 40 tons.  A modern land mammal that size would likely weigh nearly twice that and basically be too heavy to get around.  

Finally the fourth and maybe coolest reason, a mega mammal would probably cook its own tissues.  Mammals, as you probably know, are endothermic or "warm blooded".  Our bodies generate and maintain their own temperature internally.  Reptiles, meanwhile, are ectothermic or "cold blooded", absorbing heat from their environment like a snake sunning on a rock.  There's actually debate as to whether some dinosaurs were warm blooded like birds or if they had some sort of intermediate system that was neither all cold blooded nor all warm, but regardless we know a mammal uses way more energy than a similar sized reptile so it's probable that modern land mammals couldn't get much bigger than elephants without actually cooking themselves from within.  That's enough to make be perfectly happy being this size.

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