Another stab:
We know there are lifeforms that require only liquid water and a source of energy, and that they are found at ocean vents, inside rocks deep below the surface, and throughout the atmosphere. Microbes are the least affected directly by (the influence of) gravity. But they depend on the existence of a large enough planet around a star so that liquid water has an "environment"; otherwise I suppose you can entertain what life would be like inside drops of water orbiting a source of heat--supposedly a drop of water from the earth's ocean would sustain life for a while if it was orbiting the earth.
But anyhoo, gravity was and is something that larger, heavier animals have had to deal with in order to get bigger than a microbe.
So, if the earth were a smaller planet, would animals like whales be bigger or smaller? Would trees be able to grow higher, and so on? Why go to all the trouble of getting large, and so having to deal with "weight"?
So you see, I'm considering that the much longer evolutionary history of microbes is because they are "willingly" constrained by gravity--one way to consider this is as a genetic constraint as well, bacteria haven't evolved the ability to get larger (except as large colonies) but retained the "successful" strategy of staying small enough to be beyond the direct effects of gravity.