I could see only the smaller animals surviving due to their ability to hide etc...if man were around at the time, but everything I've read says that man was not around to interact with prehistoric animals.
Yes we do have a few very large mammals around now but it would appear that at least sometime in the worlds history there were many very large animals around.
If it were and abundant food supply and a lack of a dominant predator then it would follow that the average man in a million years or so would be huge. Everything I hear about prehistoric times was very large including plant life. So I still wonder why things were so damn big.
Larger animals have a tendency to be very specialized and therefore very susceptible to changes in the environment -- be that a new predator (man), the destruction or limitation of a habitat (through moving ice sheets, for example), or whatnot. Also, the larger the animal, the longer the period between generations and the fewer offspring per generation. Whereas smaller animals reproduce like it's going out of style and have relatively large numbers of offspring per generation, ensuring that at least some will make it regardless of environmental stresses, big critters like elephants have one offspring at a time and take longer to gestate and raise that offspring to reproductive age which increases the chances it won't survive that long.
Not really. During the Carboniferous period the amount of oxygen in the air was quite a bit higher, so there were large insects and ferns but relatively few large vertebrates. During the ages of the dinosaurs, most of the extent critters were actually wolf sized and smaller. Only a relatively small number of species achieved gigantic size. There were large "terror birds" right after the end of the end of that era, but most never got bigger than ostriches and moas. The largest mammal was the Indricotherium, which topped the scales at 20 tonnes, but most of its contemporaries were about the size of modern mammals.