Reasons for dinosaurs being so big?

[W3pcq]

I have always had this idea but not sure if it is real. I always thought they were so big because there was more o2 in the air, and eventually they reduced o2 levels leading evolution to favor creatures that didn't need so much of it. Is this true?

 

[NoTime]

I don't know about dinosaurs, but I came across one study that says it is true for insects.

 

[mgb_phys]

There was higher O2 in certain periods which allowed bigger insects ( insects breath by gas diffusion which limits their size). Not all dinosaurs were big, it is just easier to find and identify large fossil remains and museums + hollywood don't focus on little creatures

For plant eaters it is likely that the poor quality vegetation avaialble (woody ferns) means you need a long time to digest them, which means you need to hold a lot of 'stock' in your stomach whih means a large stomach and so a large body. The same reason we have elephants. If you have large prey, you need to be a large predator to take them on.
There are also thermal advantages to being large if you are cold blooded, it's easier to keep a large body warm in cold climates than a small one.

Interestingly population models suggest that the mass of large predator dinosaurs/area is about the same as the mass of African big cats/area, when you take out the relative efficency of mammal/reptile bodies.