The gravity question is one I've pondered myself as a thought exercise. There's been discussions on how far up a plant can draw water as the defining limit to how tall a tree could grow. Some discussions as well on how tall an animal could grow based on how high blood could be pumped up. Which is a direction different from the structural support and sizes that I find interesting.
I can imagine an intelligent species on a high-G planet scratching their “heads” and wondering, as they surveil Earth, how anything could possibly survive on such a low-G planet.
