Why were prehistoric versions of modern mammals almost always larger?

[jc_4130]

Also the reason that Africa has the most remaining megafauna is probably because that is where modern humans evolved.  The animals were able to evolve alongside them and had time to adapt. When organized humans moved elsewhere in the world, the local fauna probably had no time to adapt.  Or so the theory goes.

The question that I find interesting is: without human intervention (assuming that is the cause for these extinctions), would the dominant mammals have kept growing larger as the dinosaurs did?

[Ludodactylus]

The question that I find interesting is: without human intervention (assuming that is the cause for these extinctions), would the dominant mammals have kept growing larger as the dinosaurs did?

Now I'm imagining an Indricothere-sized "Uber-Moose."

[Newt]

Well, the whales did...

It's possible that the largest mammalian herbivores were already pretty well maxing out what their physiology could do. Warm-bloodedness takes a lot of energy, and for various reasons having to do with surface area-to-volume ratios and resource distribution, bigger herbivores must spend more of their time eating than smaller ones. Modern elephants are pretty close to being round-the-clock eaters, and the mega-elephants and indricotheres of times past must have been even closer. You can't eat more than 24 hours a day, no matter how much you want to, and you can't go back in time and evolve a sauropod-style digestive system, no matter how convenient it would be.

I'm not aware of any similar limitations on carnivores. Maybe we could have gotten elephant-sized bears or cats eventually. There are always drawbacks to getting bigger, though, even if it's not physically impossible, so who knows.

Not all mammal groups hit their top size in the Pleistocene. Rodents, canids, and walruses, for example, topped out in the Miocene and never got as big again. Can't blame those on humans, we weren't around yet!

[Faelrin]

As far as I understand the biggest issue with mammals getting bigger is because our bones are dense, and not pneumatic like in sauropods, theropods, etc. We don't have air sacs either keeping things lighter. Palaeoloxodon namadicus and Paraceratherium were probably pushing the limits for mammals on land. Being bigger also puts more work and strain on a circulatory system as well (which can be seen observed within certain populations, such as body builders, obesity, exceptionally tall). It also means more heat needing to be shed (depending on the environment. Woolly mammoths needed thick fur despite their large size because of the freezing conditions they had to endure). With whales they have much more freedom to get bigger in water, hence the Blue whale, and its relatives. I think the blue whale might also be pushing what's physiologically possible for a vertebrate as well.

Another important factor is probably ecosystem as well. Larger animals need to intake more calories to survive, so unless there's plentiful food to support a population of large animals, (and I suppose lack of predation and competition), it may hinder the sizes of animals. I think @stargatedalek also made a great point regarding that, and to add on to it most of what is around to this day are the survivors. Not particularly relevant, but perhaps worth taking a look at is evolutionary anachronisms, as it helps provide evidence of what the previous ecosystems were like, prior to the disappearance of most megafauna.

Though not mammals per se, it may be worth mentioning that some birds like the Haast's Eagle, Moa's and Aepyornis were some of the largest birds of their kinds, and survived pretty recently before our species saw to their extinctions. They would likely still be around if it weren't for early human settlers taking advantage of them.