[Swordmaker]

David Esker proposes that dinosaurs were able to grow so large because the atmosphere was many times more dense—not that gravity was in someway attenuated—dense enough that it was the equivalent of "swimming through water" which supported their bodies.

[varmintman]

Should be obvious... Make the atmosphere dense enough for anything to float on it, and a creature trying to breath such an atmosphere would burn up like one of those stupid experiments where some redneck throws liquid oxygen on a fire.