eldritchamy:

Realistic answer?  Mostly because humans hadn’t come around and hunted them all to extinction yet.  Dinosaurs are exempt from this because they vastly predated us, but almost anything that coincided with our timeline, we killed.

We are, for our relatively small size and frail, sometimes clumsy physical characteristics, a TERRIFYING species.

Evolution has produced all kinds of Big Shit.  Ever seen a Paraceratherium?

Or a size chart for Sauropods that wasn’t produced before 1970?

Evolution likes to make things big.  It tries this all the time.  Whenever there’s a plentiful food source and enough space, things just get bigger and bigger.  

The largest animal to have ever lived is alive right now.  It’s the blue whale.  And it’s truly a masterpiece of evolution’s drive to Go Bigger.

HOW THIS HAPPEN.

Basically, because they live in the ocean, space isn’t really an issue for them, and thanks to buoyancy, neither is their frankly ALARMING weight.  The only real limit to their size is chemistry – whether they can possibly metabolize enough energy fast enough to stay alive at their size.  Blue whales are estimated (having, for obvious reasons, never been measured in one piece) to be able to reach over 200 tons.  As an average weight.  Fluctuating with their feeding season.  This was for a 98 foot long whale.  The longest whales ever measured were 110 feet and 109 feet, both females.  (Males tend to be slightly shorter, but heavier at any given length).

A blue whale can hold over 90 tons of food and water in its mouth.

They need 1.5 million kilocalories of food per day.

Blue whales are MASSIVE.

They are not the LONGEST animal in the world, though, just the heaviest.  The longest is likely one of two things: the Lions Mane Jellyfish or the Bootlace Worm.  The longest recorded Lions Mane Jellyfish washed up on shore with tentacles measuring 120 feet.  It is unknown if they can be longer than this, but certainly possible given how fragile they are and the fact that this is just one that happened to get washed up on a beach.

The longest recorded bootlace worm SMASHES this record, but because of its stretchy body and the date of the recording (1864), the scientific accuracy is disputed.  It also washed up on shore and measured 180 feet.

How far off topic am I this time?

Anyway yes animals get big sometimes.  It helps deter predators when you’re too big to be hunted by anything.  The only natural predator of the blue whale is killer whales.  Regarding bears specifically, Brown Bears are in far more trouble than Black Bears because the Brown Bear line trends toward going bigger, which makes them easier targets for humans, while Black Bears have evolved to be shy and stealthy and avoid human contact.  As a result, Brown bears are far larger, but far more likely to be driven to extinction by humans.  Nature functions just fine if it’s left alone.  We just ruin everything we touch.  That’s why the largest individual crocodiles still living right now are the ones that have learned to avoid humans at all costs: conservation laws have not protected crocodiles from poaching long enough for them to get really, really big, even though we have significant historical records of crocodiles larger than what we generally see now.  At least some of those records are considered to be reliable and put a couple extant crocodile species well over 20 feet – some over 22.  The largest reliably measured crocodile was Lolong, a Saltwater crocodile in the Philippines who measured 20 feet 3 inches and died a few years ago.

And Argentavis magnificens was able to fly because it was designed to.  Even with a massive 24 foot wingspan, it only weighed around 175 pounds, because birds have very lightweight skeletons.  As impressive as the size was,

a living bird of that size probably weighed about as much, if not a bit less, than the man standing next to it.  The surface area of its wings would have been sufficient to keep it in the air, mostly by gliding the way you see large modern birds of prey do.  It would have resembled a condor or vulture, just much larger.