Because we evolve to our surroundings, and gravity would probably be a big factor in our average size, could the fact that dinosaurs were so much bigger than us be because there was less gravity then, and the asteroid added to the mass of the earth with a significant change in gravitational power? Do you think that, in the time of the dinosaurs, things would have still fell with an acceleration of 9.5ms^-2?

no. if a meteor of enough mass to change the gravity significantly hit the earth, it would very likely get knocked out of orbit. dinosaurs where big because the area was generally open plains, where large animals thrive, because you cant hide anywhere, so the only real defense is to be bigger than the rest.

in the forests, animals are usually small, as they can hide.

the reason why i dont think we get large ass land animals anymore is because the large animals are mainly mammals, which are warm blooded, which have to eat a lot more than reptiles, who maintain homeostatic temperature by sunlight. since there isnt enough food, they evolved to be a bit smaller, since smaller animals need less energy to maintain a the correct body heat.

the reason why reptiles dont rise up again is because mammals have been given such a big headstart after the mass extinction long ago that they would now just eat any lizard that dares to grow too big, so the lizards have been naturally selected to be small

though thats what i reckon. someone comfirm/deny this?

You are on the right track, for the most part. Also, there is less oxygen in the air now than there was then.

The big mammals got eaten up by our ancestors.

I call it as either evolution determined that smaller mammals survived better, or an extinction event wiped out almost all life and whatever remained evolved into what we have today.

I would have thought that if the gravity increased we would have to develop to be bigger to handle the strain of an increase of weight, but obviously that is not the case