I've a curious inquiry for some of the more science minded: how does a planet's particular gravity and atmosphere affect an organisms weight and height? For instance, what would the specific conditions have to be for a planet to have mostly large fauna? Or paradoxically, what sort of gravity and atmosphere would cause species to tend towards smaller physiques? I'm mostly curious because I'd like to work the details into some fiction I've been writing. Thanks for any help.
Not sure that gravity and atmosphere alone would be enough to control the size of species. Everything is to intermixed. In general you could say high gravity results in short and stocky with thicker limbs. But what if the high energy food source was located up high. Would evolution force a growth spurt countering the effects of the gravity?
Well, lower gravity would allow for taller creatures. Also, from watching Discovery channel I learned that a higher percentage of oxygen in the atmosphere allowed giant insects (on the order of several feet in size) to one time roam the earth. If you think about it, in our ecosystem the limiting factor is getting oxygen to parts of the body. Whether it's high gravity making it hard to pump oxygen to the upper extremities, or low oxygen content putting an upper limit on intake. I am not sure, but I am guessing there is for the most part some limiting factor as to how large lungs are in relation to the rest of the body, at least statistically speaking. The benefits of the extra intake would be outweighed by the disadvantages of the extra mass.