Star Trek

This monster scale law show if life on other plenet what it would look like by study earth history.

One theory suggests that levels of atmospheric oxygen were much higher, especially in pre-cambrian times (the era of giant insects and arthropods). Because invertebrates do not "breathe" with lungs, they absorb oxygen from the atmosphere through spiracles. If oxygen is limited (as today) then the size of the invertebrate is also limited.

Even the largest dinosaurs were smaller than modern whales, so there is no real size limitation governed by heart size, and some dinosaurs did apparently have more than one heart, (octopuses have three!)

Physical support is not such an issue as you might think, if you compare the bone structure of a modern large mammal (such as an elephant) you'll see that the bones are quite dense, which limits size due to the mass of the bone. Dinosaur bones were often quite hollow (rather similar to birds) which reduced the weight without reducing strength (try to crush a toilet roll tube vertically and you'll see what I mean) so the animal could reach a greater size, as its mass/size ratio was larger (ie physically bigger for the same weight)

Feeding was also another reason, herbivorous dinosaurs, rather than chewing their food, tended to be adapted to break off larger chunks of vegetation and swallow them whole, meaning a vastly higher biomass could be consumed to fuel large size. This meant strong teeth and jaws were not needed, so dinosaurs heads tend to be rather light (allowing long necks for example) compared to a mammals heavy jaws/teeth/musculature.

Also consider that the planet was rather more uniform than it is now, with the land masses concentrated together, meaning much larger areas of suitable food, whereas the planet as it is now is much more divided into temperate, tropical and polar regions, where plant species are more varied, so that animals are in effect "corralled" into a smaller space because of food availibility and the difficulties of crossing large oceans to reach other sources!

I must note there are a draw back being very big you have to eat lots and the plenet would have to have lots of food

The Antarctosaurus was very very big and had to eat all the time but in past earth had way more Jungle with trees very tall than today.So today the Antarctosaurus could not be alive.